Method for regulating electric double layer to realize efficient CO2 electro-reduction to form formic acid

Abstract

The present application relates to the technical field of electrochemical energy conversion for preparing high value-added products, and relates to a method for realizing high-efficiency CO2 electro-reduction for formic acid by regulating electrochemical double layer through electrolyte engineering, which adds trace quaternary ammonium salt single-chain cationic surfactant in cathode electrolyte, and under the driving of applied reduction potential, the positively charged organic cations are dynamically self-assembled on the negatively charged CuO foam electrode surface through electrostatic interaction, the electrochemical double layer of solid-liquid interface is reconstructed, the proportion of free water at the interface is reduced, and the proton supply required for the hydrogen evolution side reaction is blocked; and the quaternary ammonium cations stabilize the formic acid intermediates; experimental data show that the CuO foam electrode, device and method prepared based on the present application have a selectivity of the target product formic acid of more than 80% in the industrial-grade large current density range of 100-500 mA / cm 2 The method is simple to operate and low in cost, and provides a new path for interface microenvironment regulation for the scale industrial application of CO2 electro-reduction.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy conversion for the preparation of high-value-added products, and in particular to a method for achieving efficient CO2 electroreduction to formic acid by regulating the electrochemical double layer through electrolyte engineering. Background Technology

[0002] With the continuous advancement of global industrialization, the excessive consumption of fossil fuels has led to a sharp increase in atmospheric carbon dioxide (CO2) concentration, triggering severe environmental problems such as the greenhouse effect. To achieve the goal of "carbon neutrality," electrochemical carbon dioxide reduction (CO2RR) technology driven by renewable energy, which converts greenhouse gases into high-value-added chemicals or fuels, is considered a highly promising green energy conversion strategy. Among the many reduction products, formic acid (HCOOH) or formate is not only an important basic chemical raw material but also a safe and efficient liquid hydrogen carrier with enormous commercial application potential.

[0003] Although CO2RR has made significant progress in laboratory-scale studies, for it to truly move towards industrial applications, the system must have a high current density (greater than 200 mA / cm²) suitable for industrial applications. 2 Maintaining high energy efficiency and high product selectivity is crucial. Currently, industrial-grade CO2 electrolysis mainly employs flow electrolysis reactors. However, under high current density conditions, the cathode catalytic interface faces extremely severe challenges: on the one hand, high current density is usually accompanied by a violent hydrogen evolution reaction (HER), where excessive free water molecules at the electrode-electrolyte interface not only exacerbate hydrogen production as reactants but also severely reduce the Faradaic efficiency of the target product, formic acid; on the other hand, excessive free water and violent bubble evolution can severely hinder the effective adsorption and conversion of key reaction intermediates (such as *OCHO) on the catalyst surface.

[0004] To address the aforementioned issues, existing technological approaches primarily focus on two directions: first, developing nanostructured catalysts with complex crystal planes or defect engineering. However, this often involves extremely complex synthesis processes, making low-cost, large-scale preparation difficult, and the structures are prone to deactivation and collapse under high current. Second, adjusting the electrolyte phase composition, for example, by using expensive ionic liquids or high-concentration strong alkaline solutions. However, this inevitably leads to engineering problems such as increased system viscosity, higher costs, and severe equipment corrosion. Traditional catalyst pre-modification processes often struggle to simultaneously ensure the long-term stability of the hydrophobic interface and enhance the adsorption capacity of intermediates. Therefore, there is an urgent need in this field for a method that is simple, low-cost, and capable of effectively controlling the interfacial microenvironment under industrial-grade high current densities to overcome the bottlenecks of poor formic acid selectivity and low energy efficiency. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention proposes a method for achieving efficient CO2 electroreduction to formic acid by regulating the electrochemical double layer.

[0006] The technical solution of this invention is as follows: A method for producing formic acid by electrochemical reduction of CO2 by regulating the electrochemical double layer includes the following steps: S1. Preparation of cathode electrolyte: CO2 gas is passed into the electrolyte to obtain CO2 capture solution, quaternary ammonium salt cationic surfactant is added to the CO2 capture solution, and after mixing, cathode electrolyte is obtained; S2. Electrocatalytic reduction: The cathode electrolyte obtained in step S1 is passed into the cathode chamber of an electrolytic cell containing a cathode electrode, and current or voltage is applied to the electrolytic cell to perform CO2 electrochemical reduction to prepare formic acid.

[0007] Furthermore, the quaternary ammonium salt cationic surfactant mentioned in step S1 is a single-chain quaternary ammonium salt cationic surfactant with a carbon chain length of C4-C16.

[0008] Further, the quaternary ammonium salt cationic surfactant mentioned in step S1 is selected from at least one of butyltrimethylammonium bromide, octyltrimethylammonium bromide, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and hexadecyltrimethylammonium bromide.

[0009] Further, the electrolyte in step S1 is an aqueous solution of an alkali metal salt, wherein the cation of the alkali metal salt is selected from at least one of lithium, sodium, potassium or cesium; and the anion of the alkali metal salt is selected from at least one of bicarbonate, sulfate, chloride, bromide or iodide.

[0010] Further, the concentration of the electrolyte in step S1 is 0.1-3 M, and the concentration of the quaternary ammonium salt cationic surfactant is 0.5-2 mM.

[0011] Furthermore, the cathode electrode mentioned in step S2 is a foamed CuO electrode.

[0012] Further, the preparation method of the foamed CuO electrode includes the following steps: using foamed copper as the working electrode, performing constant current electrochemical oxidation treatment in a KOH or NaOH solution with a concentration of 1-3 mol / L; cleaning the treated electrode and drying it at 160-180℃ for 30-60 min to obtain the foamed CuO electrode, wherein the current density range is selected from 0.25-2.5 mA cm⁻¹. -2 The oxidation treatment time is selected from 3600-9600s.

[0013] Furthermore, the constant current density of the current in step S2 is 100-500 mA cm⁻¹. -2.

[0014] Furthermore, the electrolytic cell in step S2 further includes an anode chamber and a cation exchange membrane disposed between the anode chamber and the cathode chamber; The anode chamber is equipped with a foamed nickel anode electrode, and KOH or NaOH is introduced into the anode chamber as an electrolyte.

[0015] Furthermore, the selectivity of formic acid prepared by the CO2 electrochemical reduction is greater than 80%.

[0016] Compared with the prior art, the present invention has at least the following advantages: This invention relates to a method for achieving efficient CO2 electroreduction to formic acid by regulating the electrochemical double layer through electrolyte engineering. The method involves adding a trace amount of quaternary ammonium salt single-chain cationic surfactant to the cathode electrolyte. Driven by an applied reduction potential, the positively charged organic cations undergo dynamic self-assembly on the negatively charged CuO foam electrode surface through electrostatic interactions, reconstructing the electrochemical double layer at the solid-liquid interface and forming a novel organic cation-mediated electrochemical double layer structure. The long carbon chains of the surfactant form a hydrophobic barrier at the interface, displacing the originally attached high-dielectric solvent water molecules and reducing the proportion of strongly hydrogen-bonded water at the interface, thus physically blocking the proton supply required for the hydrogen evolution side reaction. Simultaneously, the quaternary ammonium cations adsorbed on the electrode surface stabilize the key formic acid intermediate (*OCHO), effectively reducing the double layer capacitance and charge transfer resistance at the interface.

[0017] Experimental data show that, when using quaternary ammonium salts with specific carbon chain lengths (such as dodecyltrimethylammonium bromide DTAB) at 0.5-2 mM, the CuO foam electrode and device prepared based on this invention achieve performance at 100-500 mA / cm². 2 Within the industrial-grade high current density range, the Faraday efficiency of the target product formic acid is above 80%. The method of this invention is simple to operate and low in cost, providing a new path for the large-scale industrial application of CO2 electroreduction by controlling the interface microenvironment. Attached Figure Description

[0018] To more clearly illustrate the specific embodiments of the present invention, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below.

[0019] Figure 1 This is a schematic diagram of the preparation method of CuO foam electrode in Example 2; Figure 2 SEM images of the Cu foam electrode and CuO foam electrode from Example 2; Figure 3 The XRD patterns of the Cu foam electrode and CuO foam electrode in Example 2 are shown. Figure 4XPS images of the Cu foam electrode and CuO foam electrode in Example 2; Figure 5 This is a schematic diagram of the CO2 electroreduction formic acid production apparatus in Example 1; Figure 6 XPS images of the CuO foam electrode in Example 3 after electrolysis in electrolyte systems containing DTAB and those without DTAB; Figure 7 The graphs show the electrochemical impedance spectroscopy results of the CuO foam electrode in Example 3 in electrolyte systems containing and without DTAB. Figure 8 The images show in-situ infrared spectra of Example 3 in two electrolyte systems, one containing DTAB and the other without. Figure 9 This is a peak fitting diagram of the interfacial water presence in Example 3 under systems containing and without DTAB electrolyte; Figure 10 The graph shows the electrochemical reduction performance of the CuO foam electrode in an electrolyte system containing DTAB, as shown in Example 4. Figure 11 This is a performance test diagram of the CuO foam electrode in Example 4, used for electrolyzing a CO2 saturated solution in an electrolyte system without DTAB. Figure 12 This is a comparison chart of the performance test results of electrolyzing a saturated CO2 solution in the presence of cationic surfactants with different carbon chain lengths in Example 4.

[0020] Explanation of reference numerals in the attached drawings: Flow meter 1, CO2 capture device 2, cathode inlet pump 3, electrolytic cell 4, cathode electrode 5, cation exchange membrane 6, anode electrode 7, anolyte storage bottle 8, anolyte pump 9. Detailed Implementation

[0021] The present invention will now be described in further detail. It should be noted that the following specific embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above application content.

[0022] This invention provides a general and / or specific description of the materials and experimental methods used in the experiments. Unless otherwise specified, all experimental or testing methods are conventional methods; all reagents or instruments used, unless otherwise specified, are commercially available conventional products prepared or used using conventional methods.

[0023] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art.

[0024] Materials and Equipment Copper foam: purchased from Kunshan Guangjiayuan Electronic Materials Business Department, 1mm thick; This invention uses an electrochemical workstation (PARSTAT MC, USA) for electrode treatment and CO2 electrochemical reduction testing; a gas chromatograph (Aglient 8890, USA) and a 400MHz nuclear magnetic resonance spectrometer (AVANCENEO 400, Switzerland) are used to test the gas-phase and liquid-phase reduction products, respectively.

[0025] Example 1: Preparation and Characterization of CuO Foam Electrode This embodiment provides a method for preparing a CuO foam electrode for a cathode. This electrode has a higher electrochemical active surface area, and its abundant interconnected pore structure facilitates the flow of electrolyte solutions containing saturated CO2. A schematic diagram of the preparation process is shown below. Figure 1 As shown, the preparation method includes the following steps.

[0026] S1. Cleaning: The 1mm thick copper foam was cleaned by ultrasonic oscillation using anhydrous ethanol, dilute hydrochloric acid and deionized water in sequence, with each step taking 15 minutes to remove surface impurities.

[0027] S2. Constant Current Oxidation: The cleaned foamed copper was used as the working electrode, and a graphite electrode as the counter electrode. Constant current electrochemical oxidation was performed in the electroplating bath. The alkaline oxidation solution was a 1 mol / L KOH solution; a constant current of 0.25 mA cm⁻¹ was applied to the working electrode. -2 The oxidation current is set at 9600 s, and the oxidation time is 9600 s. Optionally, in step S2, the alkaline oxidizing solution is selected from KOH or NaOH, the concentration range of the alkaline oxidizing solution is selected from 1-3 M, and the oxidation current range is selected from 0.25-2.5 mA cm⁻¹. -2 The oxidation time range is selected from 3600-9600s.

[0028] S3. Dehydration and Drying: After washing the oxidized copper foam electrode obtained in step S2 with deionized water, it is dehydrated and dried in air at 180°C for 60 minutes to obtain a CuO foam electrode with a high electrochemically active surface area. Compared with the Cu foam electrode that has not undergone electrochemical oxidation, this electrode greatly increases the reaction rate, and the preparation method is easy to scale up industrially.

[0029] The microstructures of Cu foam electrodes and CuO foam electrodes are as follows: Figure 2 As shown, the SEM images clearly reveal the nanostructure of the foam skeleton and surface; Figure 3 The X-ray diffraction (XRD) patterns of Cu foam electrodes and CuO foam electrodes prove that the electrodes after electrochemical oxidation treatment contain CuO. Figure 4The images show X-ray photoelectron spectroscopy (XPS) spectra of Cu foam electrodes and CuO foam electrodes. The test results indicate that the electrodes after electrochemical oxidation treatment exhibit Cu 2p... 3 / 2 (~933-934 eV) and Cu2 p 1 / 2 The main peak appears at (~953-954 eV), accompanied by significant shock satellite peaks at 941-945 eV and 961-965 eV, indicating that the sample is mainly composed of Cu. 2+ It exists in form.

[0030] Example 2: An apparatus for the efficient electroreduction of carbon dioxide to produce formic acid This embodiment provides a highly efficient apparatus for the electroreduction of carbon dioxide to formic acid, the apparatus comprising a CO2 capture module and an electrochemical reaction module; the apparatus as follows: Figure 5 As shown.

[0031] The CO2 capture module is used to prepare the cathode electrolyte. CO2 gas is dissolved in the electrolyte using a conventional bubbling method via a flow meter 1 to obtain the CO2 capture solution, ensuring sufficient dissolution of small liquid CO2 molecules. Under normal pressure, the concentration of small liquid CO2 molecules in this capture solution is approximately 33 mmol / L.

[0032] The electrochemical reaction module consists of a cathode circulation loop, an anode circulation loop, and an electrolytic cell 4.

[0033] Cathode circulation loop: The CO2 capture device 2, which contains CO2 capturing solution (containing quaternary ammonium salt cationic surfactant), is connected to the cathode inlet pump 3 through a pipeline; the output end of the cathode inlet pump 3 is connected to the cathode side inlet of the electrolytic cell 4; the cathode side outlet of the electrolytic cell 4 is connected back to the CO2 capture device 2 through a pipeline to form a circulation.

[0034] Anode circulation loop: The anolyte storage bottle 8, which contains anolyte (strong alkaline KOH or NaOH solution), is connected to the anolyte pump 9 through a pipeline; the output end of the anolyte pump 9 is connected to the anode side inlet of the electrolytic cell 4; the anode side outlet of the electrolytic cell 4 is connected back to the anolyte storage bottle 8 through a pipeline to form a circulation.

[0035] The electrolytic cell 4 is powered by a DC regulated power supply. The entire cell is assembled by tightening bolts for easy disassembly. The internal components of the electrolytic cell 4 are stacked in the following order from the cathode side to the anode side in the horizontal direction: cathode plate, cathode electrode 5, cation exchange membrane 6, anode electrode 7, and anode plate.

[0036] The cathode plate is a 7 cm × 7 cm titanium plate, which serves as both a current guide and a current collector, and is connected to the negative terminal of a DC power supply via a wire. The surface of the plate is flat and has no current guide channels.

[0037] The cathode electrode 5 is a CuO foam electrode prepared in Example 1, and the actual electrocatalytic reaction area is limited to 1 cm × 1 cm, which is closely attached to the inner side of the cathode plate.

[0038] A cation exchange membrane 6 is disposed between the cathode electrode 5 and the anode electrode 7. Cations in the anolyte pass through this membrane to provide protons to the cathode side in order to maintain charge balance.

[0039] The anode electrode 7 is made of nickel foam with a thickness of 0.3 mm, and the actual electrocatalytic reaction area is 1 cm × 1 cm, which is closely attached to the other side of the cation exchange membrane 6.

[0040] The anode plate is a 7 cm × 7 cm titanium plate, which also serves as a flow guide and current collector, and is connected to the positive terminal of a DC power supply via a wire. The inner surface of the plate, which is attached to the nickel foam (i.e., anode electrode 7), has flow channels machined on it, with grooves distributed in a serpentine pattern from bottom to top.

[0041] The flow field distribution inside the electrolytic cell 4 includes a cathode foam flow field and an anode serpentine flow field.

[0042] Cathode foam flow field: Since the cathode plate is flat and without grooves, the cathode electrolyte delivered by the cathode inlet pump 3 flows directly into and through the internal three-dimensional pores of the CuO foam electrode, and then flows out through the outlet after the reaction.

[0043] Anode serpentine flow field: The anolyte pumped by the anolyte pump 9 enters the serpentine groove of the anode plate. After being guided by the flow channel, it comes into full contact with the anode foam nickel catalyst attached to the groove and flows out from bottom to top.

[0044] By applying an electromotive force to both the cathode and anode of electrolytic cell 4, redox reactions occur at the cathode and anode respectively.

[0045] The anode reaction formula is: 4OH - -4e - →2H2O+O2.

[0046] The cathode reaction is: CO2(aq) + H2O + 2e - →HCOO - +OH - And the hydrogen evolution side reaction 2H2O + 2e - →H2+2H2O.

[0047] Example 3: Method and Characterization of Controlling the Electric Double Layer at the Cathode Electrode Interface This embodiment provides a method for regulating the double layer of the electrode interface through electrolyte engineering. This method involves adding a trace amount of quaternary ammonium salt cationic surfactant to the electrolyte. After applying an electric potential, the cationic surfactant is neatly arranged at the interface. This dynamic self-assembly of the interface structure after energization changes the adsorption of key intermediates of formic acid and the structure of interfacial water, thereby effectively improving the selectivity of formic acid products and effectively suppressing hydrogen evolution side reactions.

[0048] S1. The CO2 capture solution is a 0.5M KCl aqueous solution containing saturated CO2. A 1 mM quaternary ammonium salt cationic surfactant, dodecyltrimethylammonium bromide (DTAB), is then added to the CO2 capture solution. The CO2 capture solution is pumped to the cathode side of the electrolytic cell via a peristaltic pump for circulation. Optionally, the CO2 capture solution in step S1 is an aqueous solution of an alkali metal salt. The cation of the alkali metal salt is selected from at least one of lithium, sodium, potassium, or cesium; the anion of the alkali metal salt is selected from at least one of bicarbonate, sulfate, chloride, bromide, or iodide ions, with a concentration selected from 0.1-3 M; and the concentration of DTAB in the CO2 capture solution is selected from 0.5-2 mM.

[0049] S2. Apply a reduction potential (-0.4 V to -1.2 V vs. RHE) to the electrolytic cell. During this process, negative charges are evenly distributed on the surface of the cathode CuO foam electrode, causing the quaternary ammonium salt cationic surfactant to be uniformly adsorbed on the electrode surface through electrostatic interaction, thereby forming an organic cation-mediated electrochemical double layer at the electrode interface, resulting in a rearrangement of the interface structure.

[0050] To verify the adsorption behavior of the aforementioned quaternary ammonium salt cations on the electrode surface and their alteration of the electrochemical double layer microenvironment, specific characterization and mechanism explanations are provided in conjunction with the following figures.

[0051] XPS test results are as follows: Figure 6 As shown in the figure, a positively charged nitrogen (N+) peak, representing the characteristics of quaternary ammonium cations, was detected on the surface of the CuO foam electrode after electrolysis of the DTAB solution. This result indicates the successful adsorption of cationic surfactants at the electrode interface.

[0052] Figure 7 a represents the charge transfer resistance test at different potentials. The results show that the charge transfer internal resistance (Ra) of the electrolyte system containing DTAB is... CT The result shows that the ordered interfacial structure induced by surfactant promotes more efficient charge transfer, which is significantly lower than that of pure KCl electrolyte systems. Figure 7 b represents the double-layer capacitance at different potentials (C) dlThe fitting results show that the electrolyte system containing DTAB exhibits a lower interfacial double layer capacitance value. This may be because the cationic surfactant with long-chain hydrocarbons replaces the high-dielectric solvent water molecules and mobile supporting electrolyte ions that were originally attached to the interface, thereby effectively reducing the interfacial double layer capacitance value.

[0053] Figure 8 ab represent the in-situ infrared spectroscopy results obtained in electrolyte systems without and with DTAB, respectively. Comparing the two figures shows that, compared to the electrolyte system without DTAB, the absorption peak intensity of the formic acid intermediate is significantly increased in the electrolyte system containing DTAB, proving that the adsorption layer enhances the stability of the intermediate.

[0054] Figure 9 To determine the three forms of interfacial water, peak fitting was performed on the water peaks in the infrared test results. The fitting results show that the electrolyte system containing DTAB reduces the proportion of free water at the interface, improves the hydrophobicity of the electrode interface, reduces the proton source for the hydrogen evolution reaction, suppresses hydrogen evolution side reactions, and enhances the mass transfer of CO2 gas at the interface.

[0055] Example 4: Performance Test and Comparative Experiment of CO2 Electroreduction for Formic Acid Production This embodiment uses the CuO foam electrode of Embodiment 1, the apparatus of Embodiment 2, and the method of Embodiment 3 to conduct comparative tests on the electrolytic hydrogen production and formic acid production of the experimental group and the control group.

[0056] Experimental group: The cathode electrolyte was a CO2-saturated 0.5M KCl aqueous solution, with 1 mM DTAB added simultaneously, and the temperature was maintained at 100-500 mA cm⁻¹. -2 Performance tests were conducted within the industrial-grade high current density range, and the results are as follows: Figure 10 As shown in ab, Figure 10 a represents the product distribution and cell voltage. Figure 10 b represents the stability test. As shown in the figure, the selectivity of the target product formic acid can be maintained above 80%.

[0057] Control group: The cathode electrolyte was only a CO2-saturated 0.5M KCl aqueous solution, without the addition of DTAB, and the performance test results were as follows under the same conditions: Figure 11 As shown, the results indicate that within the range of 100-500 mA cm⁻¹ -2 Within the current density range, the products of CO2 electrochemical reduction (CO2RR) are numerous, failing to achieve high selectivity for a single product, and the hydrogen evolution side reaction (HER) is severe, consuming a large amount of charge.

[0058] Example 5: The Regulation of the Reaction by Quaternary Ammonium Salt Cationic Surfactants with Different Carbon Chain Lengths This embodiment examines the effects of quaternary ammonium salt cationic surfactants with different carbon chain lengths at 400 mA cm⁻¹. -2 The performance of CO2 electrolytic reduction under high current density was compared with that of butyltrimethylammonium bromide (BTAB), octyltrimethylammonium bromide (OTAB), dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), and hexadecyltrimethylammonium bromide (CTAB) with increasing carbon chain lengths. The test results are as follows.

[0059] Figure 12 The performance test results for electrolyzing saturated CO2 solutions are shown in the figure. As the alkyl chain length of the surfactant increases, the inhibition of the hydrogen evolution reaction (HER) gradually strengthens, thus favoring the CO2 reduction reaction and effectively improving the selectivity and Faradaic efficiency of formic acid products. In particular, the DTAB electrolyte system with a long dodecyl chain structure exhibits the most significant hydrogen inhibition effect and the best formic acid formation performance. This fully demonstrates that DTAB with a specific chain length has optimal microscopic spatial adaptability and a stronger reaction-promoting effect in regulating the electrode / electrolyte interface microenvironment and promoting CO2RR formic acid production.

[0060] In summary, this invention employs an electrolyte engineering strategy to regulate the electrode interface double layer, achieving targeted control of the electrochemical reduction of CO2 to formic acid. Applying this strategy to a CO2 capture solution system offers significant advantages such as ease of operation, low cost, and strong controllability. Furthermore, the CuO foam electrode prepared in this invention is not only simple to prepare and easily scalable, but also possesses a large specific surface area, exhibiting high catalytic activity and a high reaction rate in the electrolysis of CO2 capture solutions.

[0061] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention.

Claims

1. A method for producing formic acid by electrochemical reduction of CO2 by regulating the electrochemical double layer, characterized in that, Includes the following steps: S1. Preparation of cathode electrolyte: CO2 gas is passed into the electrolyte to obtain a CO2 capturing solution, and a quaternary ammonium salt cationic surfactant is added to the CO2 capturing solution. After mixing, the cathode electrolyte is obtained. S2. Electrocatalytic reduction: The cathode electrolyte obtained in step S1 is passed into the cathode chamber of an electrolytic cell containing a cathode electrode, and current or voltage is applied to the electrolytic cell to perform CO2 electrochemical reduction to prepare formic acid.

2. The method according to claim 1, characterized in that, The quaternary ammonium salt cationic surfactant mentioned in step S1 is a single-chain quaternary ammonium salt cationic surfactant with a carbon chain length of C4-C16.

3. The method according to claim 2, characterized in that, The quaternary ammonium salt cationic surfactant mentioned in step S1 is selected from at least one of butyltrimethylammonium bromide, octyltrimethylammonium bromide, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and hexadecyltrimethylammonium bromide.

4. The method according to claim 1, characterized in that, The electrolyte in step S1 is an aqueous solution of an alkali metal salt, wherein the cation of the alkali metal salt is selected from at least one of lithium, sodium, potassium or cesium; and the anion of the alkali metal salt is selected from at least one of bicarbonate, sulfate, chloride, bromide or iodide.

5. The method according to claim 1, characterized in that, The concentration of the electrolyte in step S1 is 0.1-3 M, and the concentration of the quaternary ammonium salt cationic surfactant is 0.5-2 mM.

6. The method according to claim 1, characterized in that, The cathode electrode mentioned in step S2 is a foamed CuO electrode.

7. The method according to claim 6, characterized in that, The preparation method of the foamed CuO electrode includes the following steps: using foamed copper as the working electrode, performing constant current electrochemical oxidation treatment in a KOH or NaOH solution with a concentration of 1-3 mol / L; cleaning the treated electrode and drying it at 160-180℃ for 30-60 min to obtain the foamed CuO electrode; the current density range is selected from 0.25-2.5 mA cm⁻¹. -2 The oxidation treatment time is selected from 3600-9600s.

8. The method according to claim 1, characterized in that, The constant current density of the current in step S2 is 100-500 mA cm⁻¹ -2 .

9. The method according to any one of claims 1-8, characterized in that, The electrolytic cell in step S2 further includes an anode chamber and a cation exchange membrane disposed between the anode chamber and the cathode chamber; The anode chamber is equipped with a foamed nickel anode electrode, and KOH or NaOH is introduced into the anode chamber as an electrolyte.

10. The method according to claim 9, characterized in that, The formic acid prepared by the electrochemical reduction of CO2 has a selectivity of greater than 80%.

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