Method for converting aluminum electrolysis tail gas into sulfur-nitrogen co-doped composite carbon material in one step

Abstract

The invention relates to a method for converting aluminum electrolysis tail gas into a sulfur-nitrogen co-doped composite carbon material in one step, and belongs to the technical field of pollution gas conversion and application. According to the method, the factory electrolysis tail gas is used as a raw material, and the factory electrolysis tail gas is converted into the high-value sulfur-nitrogen co-doped composite carbon material through a fused salt electrolysis method. An electrolytic molten salt system is a quaternary mixed system of lithium chloride, potassium chloride, potassium hydroxide and sodium borate, the sulfur-nitrogen co-doped composite carbon material is flaky or tubular and has good electrochemical performance, and the specific capacity is about 588 mAh / g when the sulfur-nitrogen co-doped composite carbon material is tested under the current density of 500 mA / g; and the capacity retention ratio reaches up to 95.47% after 500 times of circulation under the current density of 500mA / g. The waste carbon dioxide is converted into the high-quality carbon material by regulating and controlling the electrolysis process and the fused salt components, and the prepared carbon material has high specific capacity, high conductivity and high cycle stability and is suitable for various energy storage devices such as lithium ion batteries, sodium batteries and supercapacitors.

Description

TECHNICAL FIELD

[0001] The application relates to a method for one-step conversion of aluminum electrolysis tail gas into sulfur-nitrogen co-doped composite carbon materials, and belongs to the technical field of pollution gas conversion and application. BACKGROUND

[0002] The main components of aluminum electrolysis tail gas are carbon dioxide (CO2), nitrogen oxides (NO x , mainly NO2) and sulfur dioxide (SO2), as well as a small amount of harmful substances such as HF and dust. The resource utilization of CO2, NO2 and SO2 and other greenhouse gases and acid gases in aluminum electrolysis tail gas not only can reduce the total carbon emission and alleviate the emission pressure of pollutants, but also can give them secondary value and promote the green transformation and resource recycling of the metallurgical industry. Traditional gas purification methods rely on physical adsorption, chemical absorption or catalytic conversion, etc. The equipment is complex, the energy consumption is high, the efficiency is limited, and it is difficult to realize the synchronous treatment and high-value utilization of multiple components.

[0003] As a method for electrochemical conversion of CO2, nitrogen oxides and other gases, molten salt electrolysis has attracted attention due to its high efficiency, energy saving and environmental friendliness. In the process of molten salt electrolysis, gases are reduced to valuable products (such as solid carbon and oxygen or carbon-based fuels) at relatively low temperatures by applying electrical energy, realizing the resource utilization and recycling of carbon. Previous studies have shown that CO2 can form carbonate ions (CO3 2- ) in molten salt, which can be reduced to carbon deposits through electrochemical reduction; NO2 and SO2 can be converted into nitrate (NO3 - ) / nitrite (NO2 - ) and sulfite (SO3 2- ), respectively, as active nitrogen and sulfur sources to participate in doping reactions. However, current research based on molten salt electrolysis mainly focuses on the reduction of a single gas (such as pure CO2) or carbon deposition. In industry, tail gas is usually a mixture of complex components, and the reaction behavior of mixed gases is complex. The competition and synergy of doping components significantly affect the structure and performance of the final product, so it is necessary to systematically study the parameters such as molten salt composition, gas composition and structure inducer required for electrochemical conversion of mixed gases, in order to realize the controllable synthesis of sulfur-nitrogen co-doped composite carbon materials.

[0004] In addition, although traditional molten salt electrolysis method can convert carbon dioxide into carbon materials such as graphite, activated carbon, electrochemical graphene, etc., which have good stability and electrical conductivity, their specific capacity, ion diffusion or surface activity are limited, which is difficult to meet the demand of next-generation high-performance energy storage devices. SUMMARY

[0005] To address existing technical issues such as environmental pollution from industrial tail gas, low resource utilization, and complex carbon material preparation processes, which rely on exogenous dopants, the present invention provides a method for converting aluminum electrolysis tail gas into a sulfur-nitrogen co-doped composite carbon material in one step. Using factory electrolysis tail gas as raw material, the method utilizes molten salt electrolysis to convert the factory electrolysis tail gas into a high-value sulfur-nitrogen co-doped composite carbon material. This method integrates key technologies such as tail gas purification, molten salt system regulation, electrochemical deposition, and morphology induction, achieving in-situ synergistic doping of carbon, nitrogen, and sulfur sources and the simultaneous construction of graphite structures, making it suitable for the large-scale preparation of high-performance energy storage materials.

[0006] A method for converting aluminum electrolysis tail gas into a sulfur-nitrogen co-doped composite carbon material in one step, comprising the following specific steps: (1) The aluminum electrolysis tail gas is subjected to preliminary dehumidification, scrubbing, dust removal, and drying treatments in sequence to obtain purified electrolysis tail gas, which is a mixed gas of CO2, NO2, and SO2. Specifically, the aluminum electrolysis tail gas is subjected to heat exchange in a cooling heat exchanger (20-30°C) for preliminary dehumidification, passed into a first-stage wet spray scrubber for a scrubbing (the spray liquid is deionized water or NaHCO3), then defogged by a demister, and then passed into a second-stage wet spray scrubber for scrubbing (the washing buffer is Na2HPO4 / NaH2PO4), passed into a ceramic membrane filter or a bag dust collector for fine dust removal, and finally passed into a gas dryer for dehumidification to reduce the relative humidity to <5%; (2) LiCl and KCl are configured into a eutectic molten salt matrix, KOH is added to form a basic molten salt electrolyte system, and the structural inducer NaBO2 is added to the end of the basic molten salt electrolyte system, and mixed evenly to obtain a LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt system; (3) Under an inert atmosphere, the LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt system is heated to 250-450°C and kept at this temperature for 4-6 hours, and then heated to 550-750°C and kept warm to melt to form a LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt melt; (4) With a porous conductive electrode as the cathode and an inert electrode as the anode, the purified electrolytic tail gas is uniformly introduced into a LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt melt, and constant voltage DC electrolysis is performed at a temperature of 550-750°C, so that CO2 in the purified electrolytic tail gas is in situ reduced and deposited at the cathode and simultaneously cooperates with nitrogen and sulfur active substances to form CN and CS bonds, forming a sulfur-nitrogen co-doped composite carbon material with a controllable content; (5) After the electrolysis is completed, the cathode deposit is taken out, the molten salt residue and non-carbon impurities are removed, and the product is dried to obtain a sulfur-nitrogen co-doped composite carbon material.

[0007] Preferably, the concentration of the sodium bicarbonate solution in the first-level spraying in step (1) is 0.02-0.1 mol / L, and the concentration of Na2HPO4 in the second-level washing buffer is 0.01-0.05 mol / L.

[0008] Preferably, in step (2), the molar ratio of LiCl to KCl is 1:1-3, the amount of KOH added is 5-10 wt.% of the eutectic molten salt matrix, and the amount of NaBO2 added is 0.5-5 wt.% of the eutectic molten salt matrix.

[0009] Preferably, the porous electrode in step (4) is a porous nickel electrode, a porous graphite electrode, a conductive ceramic electrode or a carbon felt electrode, and the inert electrode is a graphite rod electrode, a platinum electrode, a ruthenium oxide electrode or an iridium oxide electrode.

[0010] Preferably, the rate of introduction of the electrolytic tail gas for purification in step (4) is 30-80 mL / min.

[0011] Preferably, the voltage of the constant voltage direct current electrolysis in step (4) is 2.5-4.0 V, and the time is 3-6 h.

[0012] Preferably, in the sulfur-nitrogen co-doped composite carbon material in step (4), the nitrogen doping amount is 2-6 wt.%, and the sulfur doping amount is 0.5-3 wt.%.

[0013] The beneficial effects of the present invention are: (1) The present invention realizes the in-situ simultaneous utilization of CO2, NO2, and SO2 in aluminum electrolysis tail gas for the first time, avoiding the introduction of additional carbon sources, nitrogen sources, and sulfur sources in the traditional carbon production process, reducing costs and environmental pollution, and achieving resource-environment coordinated control; (2) The present invention uses KOH to regulate the alkalinity and conversion balance, achieving stable introduction of nitrogen and sulfur ions and ensuring precise controllable content of doping components, which is superior to the uncontrollable nature of conventional gas-phase mixed doping strategies; (3) The present invention introduces NaBO2 to induce the formation of a multi-scale structure (sheet or tubular), which not only ensures the stability of the structure but also improves the energy storage active area, ion diffusion path and specific surface area, and has a high degree of designability for the product structure; (4) The present invention combines the advantages of heterojunction carbon composite carbon materials. The prepared carbon materials have high specific capacity, high electrical conductivity, and high cycle stability. They are suitable for various energy storage devices such as lithium-ion batteries, sodium batteries, and supercapacitors, and have significant engineering application potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is the XRD pattern of the sulfur-nitrogen co-doped composite carbon material of Example 1; Figure 2 This is the Raman graph of the sulfur-nitrogen co-doped composite carbon material of Example 1; Figure 3 This is a microscopic morphology of the sulfur-nitrogen co-doped composite carbon material of Example 1; Figure 4 The cycling performance of the sulfur-nitrogen co-doped composite carbon material as a lithium-ion battery anode in Example 1 is shown; Figure 5 This is the XPS spectrum of the sulfur-nitrogen co-doped composite carbon material of Example 2; Figure 6 This is the first charge-discharge curve of the sulfur-nitrogen co-doped composite carbon material as a lithium-ion battery anode material in Example 2; Figure 7 This is the first charge and discharge curve of the sulfur-nitrogen co-doped composite carbon material as the anode material of a lithium-ion battery in Example 3. DETAILED DESCRIPTION

[0015] The present invention will be further described in detail below in conjunction with specific embodiments, but the protection scope of the present invention is not limited to the contents described above.

[0016] Example 1: In this example, the exhaust gas from aluminum electrolysis contains 65 vol% CO2, 3 vol% NO2, 2 vol% SO2, 2 vol% CO, 1 vol% hydrogen fluoride, 10 vol% water vapor, and 17 vol% nitrogen and other inert gases. A method for converting aluminum electrolysis tail gas into a sulfur-nitrogen co-doped composite carbon material in one step, comprising the following specific steps: (1) Condensation dehumidification: The exhaust gas is cooled to 25°C through a heat exchanger to remove most of the water vapor; First-stage spraying: The tail gas enters the spray tower and is subjected to gas-liquid scrubbing with a sodium bicarbonate solution with a concentration of 0.025 mol / L to remove acidic impurity gases such as HF and CO, while achieving coarse dust removal; Secondary spray: The tail gas then enters the secondary spray tower, where 0.02 mol / L Na2HPO4 buffer (pH approximately 7.4) is used to further absorb residual acid mist and adjust the gas humidity; Filtration and dust removal: After passing through the demister, it passes through the high-temperature bag dust collector to ensure that the particle concentration is less than 5 mg / m 3 ; Drying treatment: The above tail gas enters the gas dryer to reduce the relative humidity to <5% to prevent the spraying from bringing moisture into the molten salt; Gas storage buffer: After treatment, purified tail gas is obtained, and the tail gas components are CO2 accounting for 87vol%, NO2 accounting for 7vol%, and SO2 accounting for 6vol% for subsequent electrolysis; (2) LiCl and KCl (the molar ratio of LiCl to KCl is 1:1) are configured into a eutectic molten salt matrix, KOH is added (the amount of KOH added is 5 wt.% of the eutectic molten salt matrix) to form a basic molten salt electrolyte system, and the structural inducer NaBO2 (the amount of NaBO2 added is 2 wt.% of the eutectic molten salt matrix) is added to the end of the basic molten salt electrolyte system, and the mixture is evenly mixed to obtain a LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt system; (3) Under an argon atmosphere, the LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt system was heated to 360°C and kept at this temperature for 4.5 hours to fully remove moisture and impurities in the molten salt system, and then heated to 650°C and kept melted for 6 hours to form a LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt melt; (4) Using a porous conductive electrode (a porous nickel electrode with a porosity of 65%) as the cathode and an inert electrode (graphite electrode) as the anode, the purified electrolytic tail gas was uniformly introduced into the LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt melt at a rate of 40 mL / min, and constant voltage DC electrolysis was carried out at a temperature of 550 ° C (electrolysis voltage of 2.5 V, time of 6 h), so that the CO2 in the purified electrolytic tail gas was in situ reduced and deposited at the cathode and simultaneously cooperated with nitrogen and sulfur active substances to form CN and CS bonds, forming a sulfur-nitrogen co-doped composite carbon material with a controllable content; (5) After the electrolysis is completed, the cathode deposit is taken out and dissolved with 0.5 mol / L dilute hydrochloric acid to remove the molten salt residue and non-carbon impurities, then washed with deionized water and vacuum dried to obtain a sulfur-nitrogen co-doped composite carbon material; The XRD pattern of the sulfur-nitrogen co-doped composite carbon material of this embodiment is shown in FIG. Figure 1 , a strong and sharp diffraction peak appears at about 26.5°, corresponding to the typical (002) crystal plane of graphitic carbon materials, indicating that there is a highly ordered graphitized structure in the sample with good layered arrangement and high crystallinity; there is a broad diffraction peak below 20°, which is related to amorphous carbon or structural defects; The Raman diagram of the sulfur-nitrogen co-doped composite carbon material of this embodiment is shown in FIG. Figure 2 , about 1350 cm -1 and 1580 cm -1 There are obvious D band and G band peaks at the same time, which correspond to the structural defects and graphitization degree of the carbon material respectively; the D band originates from sp 2 The edge or defect area of ​​hybrid carbon indicates that there is a certain degree of disordered structure in the sample, while the G band represents the ordered graphite structure in the carbon skeleton; the two peaks have similar intensities, indicating that the sample has a high degree of defects and a medium degree of graphitization, which is a typical partially graphitized carbon material; in addition, at 2500-3200 cm -1Weaker Raman signals can be observed in the region, which may contain 2D bands and D+G bands, indicating that a small amount of multilayer graphene or layered structure may exist in the conversion product; The micromorphology of the sulfur-nitrogen co-doped composite carbon material of this embodiment is shown in FIG. Figure 3 , sulfur-nitrogen co-doped composite carbon materials have block and tubular structures, and the nitrogen element is evenly distributed on the surface and in the gaps of the carbon material; The prepared carbon material, conductive agent (Super P), and binder (PVDF) were weighed in a weighing bottle according to a mass ratio of 80:10:10. After magnetic stirring at 400 r / min for 10 minutes, 1 ml of monomethylpyrrolidone (NMP) solution was added, and magnetic stirring was continued at 400 r / min for 5 hours before coating. The coating thickness was 100 μm. After blowing and vacuum drying for 10 hours, the positive electrode material was punched out to obtain a positive electrode sheet. In a glove box, a lithium sheet was used as a counter electrode and the above-mentioned positive electrode sheet was used as a working electrode to assemble a 2032 type button battery in a glove box. After standing for 10 hours, the cycle performance test was carried out on a Xinwei tester. The test results are as follows: Figure 4 As shown in the figure, after 500 charge and discharge cycles at a current density of 0.1 A / g, the material still maintains a stable specific capacity of about 600 mAh / g, with almost no capacity decay, indicating that the material has excellent cycle stability and electrochemical reversibility.

[0017] Example 2: In this example, the exhaust gas from aluminum electrolysis contains 68 vol% CO2, 3.5 vol% NO2, 4 vol% SO2, 2 vol% CO, 2.5 vol% hydrogen fluoride, 10 vol% water vapor, and 10 vol% nitrogen and other inert gases. A method for converting aluminum electrolysis tail gas into a sulfur-nitrogen co-doped composite carbon material in one step, comprising the following specific steps: (1) Condensation dehumidification: The exhaust gas is cooled to 25°C through a heat exchanger to remove most of the water vapor; First-stage spraying: The tail gas enters the spray tower and is subjected to gas-liquid scrubbing with a 0.05 mol / L sodium bicarbonate solution to remove acidic impurity gases such as HF and CO, while also achieving coarse dust removal. Secondary spray: The tail gas then enters the secondary spray tower, where 0.04 mol / L Na2HPO4 buffer (pH ≈7.6) is used to further absorb residual acid mist and adjust the gas humidity; Filtration and dust removal: After passing through the demister, it passes through the high-temperature bag dust collector to ensure that the particle concentration is less than 4 mg / m 3 ; Drying treatment: The above tail gas enters the gas dryer to reduce the relative humidity to <5% to prevent the spraying from bringing moisture into the molten salt; Gas storage buffer: After treatment, purified tail gas is obtained, and the tail gas components are CO2 accounting for 91vol%, NO2 accounting for 6vol%, and SO2 accounting for 3vol% for subsequent electrolysis; (2) LiCl and KCl (the molar ratio of LiCl to KCl is 1:2) are configured into a eutectic molten salt matrix, KOH is added (the amount of KOH added is 5 wt.% of the eutectic molten salt matrix) to form a basic molten salt electrolyte system, and the structural inducer NaBO2 (the amount of NaBO2 added is 1 wt.% of the eutectic molten salt matrix) is added to the end of the basic molten salt electrolyte system, and the mixture is evenly mixed to obtain a LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt system; (3) Under an argon atmosphere, the LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt system was heated to 220°C and kept at this temperature for 4.5 hours to fully remove moisture and impurities in the molten salt system, and then heated to 720°C and kept melted for 5 hours to form a LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt melt; (4) Using a porous conductive electrode (a porous nickel foam electrode with a porosity of 70%) as the cathode and an inert electrode (ruthenium oxide) as the anode, the purified electrolytic tail gas was uniformly introduced into the LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt melt at a rate of 35 mL / min, and constant voltage DC electrolysis was carried out at a temperature of 720°C (electrolysis voltage of 3.5 V, time of 4 h), so that the CO2 in the purified electrolytic tail gas was in situ reduced and deposited at the cathode and simultaneously cooperated with nitrogen and sulfur active substances to form CN and CS bonds, forming a sulfur-nitrogen co-doped composite carbon material with a controllable content; (5) After the electrolysis is completed, the cathode deposit is taken out and dissolved with 0.5 mol / L dilute hydrochloric acid to remove the molten salt residue and non-carbon impurities, then washed with deionized water and vacuum dried to obtain a sulfur-nitrogen co-doped composite carbon material; The XPS spectrum of the sulfur-nitrogen co-doped composite carbon material of this embodiment is shown in Figure 5 In the figure, the diffraction peaks at 398.4 eV, 400.1 eV, and 401.3 eV correspond to graphitic nitrogen (N-6), pyrrolic nitrogen (N-5), and nitrogen oxide or quaternary ammonium nitrogen (NQ), respectively. The N-6 peak indicates that nitrogen atoms are embedded in the carbon skeleton in the form of graphitic nitrogen, which is beneficial to improving the electronic conductivity and structural stability of the material. The N-5 peak corresponds to pyrrolic nitrogen, which is usually located in a five-membered ring structure and provides abundant reaction sites for improving the electrochemical activity of the material. The NQ peak is located at a higher binding energy, indicating that the nitrogen atoms are in a high oxidation state, which may be derived from edge oxidation or surface modification. The rate performance of the sulfur-nitrogen co-doped composite carbon material in this embodiment is as follows: Figure 6 As shown (electrode preparation, battery assembly and testing are the same as in Example 1), the sample was charged and discharged for the first three cycles in the voltage range of 0.01–3.0 V. Figure 6 The first discharge capacity is about 950 mAh / g, and the charge capacity is about 620 mAh / g, which is mainly attributed to the formation of the solid electrolyte interface (SEI) film and the occurrence of irreversible reactions; the charge and discharge curves of the second and third cycles are almost identical, the voltage platform is stable, and the capacity reversibility is good, indicating that the electrode material performs well in active site utilization and structural stability, and has good cycle stability and electrochemical reaction reversibility.

[0018] Example 3: In this example, the exhaust gas from aluminum electrolysis contains 70 vol% CO2, 4 vol% NO2, 2 vol% SO2, 3 vol% CO, 1 vol% hydrogen fluoride, 8 vol% water vapor, and 12 vol% nitrogen and other inert gases. A method for converting aluminum electrolysis tail gas into a sulfur-nitrogen co-doped composite carbon material in one step, comprising the following specific steps: (1) Condensation dehumidification: The exhaust gas is cooled to 25°C through a heat exchanger to remove most of the water vapor; First-stage spraying: The tail gas enters the spray tower and is subjected to gas-liquid scrubbing with a 0.1 mol / L sodium bicarbonate solution to remove acidic impurity gases such as HF and CO, while also achieving coarse dust removal. Secondary spray: The tail gas then enters the secondary spray tower, where 0.05 mol / L Na2HPO4 buffer (pH ≈ 7.8) is used to further absorb the residual acid mist and adjust the gas humidity; Filtration and dust removal: After passing through the demister, it passes through the high-temperature bag dust collector to ensure that the particle concentration is less than 4 mg / m 3 ; Drying treatment: The above tail gas enters the gas dryer to reduce the relative humidity to <5% to prevent the spraying from bringing moisture into the molten salt; Gas storage buffer: After treatment, purified tail gas is obtained, and the tail gas components are CO2 accounting for 95vol%, NO2 accounting for 3vol%, and SO2 accounting for 2vol% for subsequent electrolysis; (2) LiCl and KCl (the molar ratio of LiCl to KCl is 1:3) are configured into a eutectic molten salt matrix, KOH is added (the amount of KOH added is 10wt.% of the eutectic molten salt matrix) to form a basic molten salt electrolyte system, and the structural inducer NaBO2 (the amount of NaBO2 added is 5wt.% of the eutectic molten salt matrix) is added to the end of the basic molten salt electrolyte system, and the mixture is evenly mixed to obtain a LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt system; (3) Under an argon atmosphere, the LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt system was heated to 450°C and kept at this temperature for 4 hours to fully remove moisture and impurities in the molten salt system, and then heated to 750°C and kept at this temperature for 4 hours to form a LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt melt; (4) Using a porous conductive electrode (porous graphite electrode with a porosity of 80%) as the cathode and an inert electrode (iridium oxide electrode) as the anode, the purified electrolytic tail gas was uniformly introduced into the LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt melt at a rate of 80 mL / min, and constant voltage DC electrolysis was carried out at a temperature of 750 ° C (electrolysis voltage of 4 V, time of 4 h), so that the CO2 in the purified electrolytic tail gas was in situ reduced and deposited at the cathode and simultaneously cooperated with nitrogen and sulfur active substances to form CN and CS bonds, forming a sulfur-nitrogen co-doped composite carbon material with a controllable content; (5) After the electrolysis is completed, the cathode sediment is taken out and dissolved with 0.5 mol / L dilute hydrochloric acid to remove the molten salt residue and non-carbon impurities, and then washed with deionized water and vacuum dried to obtain a sulfur-nitrogen co-doped composite carbon material; the electrochemical properties of the prepared carbon material are as follows: Figure 7 As shown (electrode preparation, battery assembly and testing are the same as in Example 1), it can be seen from the figure that the first discharge capacity of the material in the voltage range of 0.01–3.0 V is close to 900 mAh / g, and the charge capacity is about 620 mAh / g. The irreversible capacity loss in the first cycle is mainly attributed to electrolyte decomposition, side reactions and the formation of SEI film; starting from the second cycle, the charge and discharge curves basically overlap, the voltage platform is stable, and the capacity hysteresis phenomenon is significantly reduced, indicating that the electrode reaction has good reversibility and stability; the material shows a good balance between high capacity and cycle stability, showing its potential in high-performance battery applications.

[0019] The above describes the specific embodiments of the present invention in detail, but the present invention is not limited to the above embodiments. Various changes can be made within the knowledge of ordinary technicians in this field without departing from the scope of the present invention.

Claims

1. A method for converting aluminum electrolysis tail gas into sulfur-nitrogen co-doped composite carbon material in one step, characterized in that: The specific steps are as follows: (1) Aluminum electrolysis tail gas is sequentially subjected to preliminary dehumidification, gas washing, dust removal, and drying to obtain purified electrolysis tail gas, wherein the purified electrolysis tail gas is a mixed gas of CO2, NO2, and SO2; (2) LiCl and KCl are configured into a eutectic molten salt matrix, KOH is added to form a basic molten salt electrolyte system, and the structural inducer NaBO2 is added to the basic molten salt electrolyte system and mixed evenly to obtain a LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt system; (3) Under an inert atmosphere, the LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt system is heated to 250-450°C and kept at a constant temperature for 4-6 hours, and then heated to 550-750°C and kept warm to melt to form a LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt melt; (4) Using a porous conductive electrode as the cathode and an inert electrode as the anode, the purified electrolytic tail gas is uniformly introduced into a LiCl-KCl-KOH-NaBO2 quaternary electrolytic molten salt melt, and constant voltage DC electrolysis is performed at 550-750°C, so that CO2 in the purified electrolytic tail gas is in situ reduced and deposited at the cathode and simultaneously cooperates with nitrogen and sulfur active substances to form CN and CS bonds, forming a sulfur-nitrogen co-doped composite carbon material with a controllable content; (5) After the electrolysis is completed, the cathode deposit is taken out, the molten salt residue and non-carbon impurities are removed, and the product is dried to obtain a sulfur-nitrogen co-doped composite carbon material.

2. The method for converting aluminum electrolysis tail gas into sulfur-nitrogen co-doped composite carbon material in one step according to claim 1, characterized in that: Calculated by volume fraction, it can purify CO285~95%, NO21.5~8%, and SO21.5~7% in electrolytic tail gas.

3. The method for converting aluminum electrolysis tail gas into sulfur-nitrogen co-doped composite carbon material in one step according to claim 1, characterized in that: In step (2), the molar ratio of LiCl to KCl is 1:1-3, the amount of KOH added is 5-10 wt.% of the eutectic molten salt matrix, and the amount of NaBO2 added is 0.5-5 wt.% of the eutectic molten salt matrix.

4. The method for converting aluminum electrolysis tail gas into sulfur-nitrogen co-doped composite carbon material in one step according to claim 1, characterized in that: In step (4), the porous conductive electrode is a porous nickel electrode, a porous graphite electrode, a conductive ceramic electrode or a carbon felt electrode, and the inert electrode is a graphite rod electrode, a platinum electrode, a ruthenium oxide electrode or an iridium oxide electrode.

5. The method for converting aluminum electrolysis tail gas into sulfur-nitrogen co-doped composite carbon material in one step according to claim 1, characterized in that: The rate of introduction of the purified electrolytic tail gas in step (4) is 30-80 mL / min.

6. The method for converting aluminum electrolysis tail gas into sulfur-nitrogen co-doped composite carbon material in one step according to claim 1, characterized in that: The voltage of step (4) constant voltage direct current electrolysis is 2.5-4.0 V, and the time is 3-6 h.

7. The method for converting aluminum electrolysis tail gas into sulfur-nitrogen co-doped composite carbon material in one step according to claim 1, characterized in that: In step (4), the nitrogen doping amount in the sulfur-nitrogen co-doped composite carbon material is 2-6 wt.%, and the sulfur doping amount is 0.5-3 wt.%.

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