A flexible porous carbon nanofiber membrane electrode and a preparation method and application thereof

Abstract

The present application relates to the technical field of seawater desalination materials, and particularly relates to a flexible porous carbon nanofiber membrane electrode, a preparation method and application thereof, wherein polyacrylonitrile and a non-metal oxide used as a porogen are used as precursors, and through an electrostatic spinning, carbonization and alkaline solution etching process, a flexible porous carbon nanofiber membrane electrode with a hierarchical porous structure is obtained, the mass ratio of polyacrylonitrile to the non-metal oxide is 3:1-10:1, and the method is a simple, controllable and easy-to-promote method for preparing the flexible porous carbon nanofiber membrane electrode, the prepared self-supporting porous carbon nanofiber material has good mechanical properties and can be freely bent and cut; meanwhile, the material has rich micropores and mesopores, the hierarchical porous structure has a high specific surface area, and thus the desalination performance of the electrode material is obviously improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of seawater desalination materials, and particularly to a flexible porous carbon nanofiber membrane electrode, a preparation method therefor and an application thereof. BACKGROUND

[0002] Nowadays, with the continuous development of global economy and rapid growth of population, the environmental pollution caused by the secondary utilization of a large amount of municipal wastewater and industrial wastewater has resulted in that more than 750 million people in the world do not have clean drinking water, and it is predicted that 1.8 billion people in the world will be in a state of fresh water shortage by 2025. In addition, 98% of the total water in the world is seawater and brackish water, so it is considered that the conversion of abundant seawater into usable fresh water is an effective way to solve the problem of fresh water resource shortage. In order to obtain fresh water from seawater or wastewater, people have developed various desalination technologies, including electrodialysis, multi-stage flash evaporation, ion exchange and reverse osmosis. However, due to the bottlenecks of low efficiency, high energy consumption, high cost and secondary pollution, the practical application of these traditional methods is severely limited.

[0003] With more and more people realizing the problem of global fresh water resource shortage, as a new desalination technology, capacitive deionization (CDI) is favored by researchers due to its advantages of low cost, high efficiency, simple operation and no secondary pollution. In a CDI cell, salt ions in the feed solution can be stored in the electric double layer of two porous carbon electrodes under the application of voltage. When the adsorption process reaches equilibrium, the salt ions can be released back into the feed solution by removing the voltage or applying a reverse voltage, so as to regenerate the CDI electrode, so the mechanism of CDI mainly depends on the electric double layer (EDLC) formed between the pore wall of the electrode material and the salt solution. In this CDI cell, the electrode material plays a crucial role, and the performance of the electrode material as the core of the CDI technology fundamentally determines the seawater desalination capacity of the CDI cell. The EDLC theory considers that the electrode material with large specific surface area and high electrical conductivity is more conducive to the CDI system, for example, activated carbon, carbon aerogel, graphene and their composites. Although the overall energy consumption of the above-mentioned carbon electrodes is low, these electrode materials have low salt adsorption capacity and unsuitability in high-salt water.

[0004] CN113772791A mentions a preparation method of a self-supporting porous carbon fiber material. First, a precursor fiber containing SnCl2 is obtained by electrospinning, then a porous Sn-carbon nanofiber composite material is obtained through a pre-oxidation, carbonization and reduction process, and finally the metal Sn in the fiber is removed by acid treatment to increase the specific surface area of the carbon fiber material and improve the specific capacitance of the electrode. As a CDI electrode material, the porous carbon fiber material can well adsorb heavy metal ions in wastewater. The detailed steps are as follows:

[0005] Step 1: SnCl2 was added to a mixed solution of polyacrylonitrile, polyvinylpyrrolidone and N,N-dimethylformamide to obtain a precursor solution. (The mass-volume ratio of polyacrylonitrile, polyvinylpyrrolidone, N,N-dimethylformamide and SnCl2 is 0.8-1.0 g: 0.2-0.4 g: 9-11 mL: 0.8-1.0 g). The mixed solution of polyacrylonitrile, polyvinylpyrrolidone, N,N-dimethylformamide and SnCl2 was transferred to a syringe with a needle made of conductive metal material, the needle was connected to a high-voltage power supply, a constant voltage of 18-21 kilovolts was applied to the tip of the needle, the pushing rate was 0.04±0.005 ml / h, the distance between the tip of the stainless steel needle and the collector was 15-18 cm, and the electrospinning was carried out under the conditions of humidity 70±5%, temperature 25±5℃ to obtain an initial nanofiber membrane.

[0006] Step 2: The initial nanofiber membrane was sequentially subjected to pre-oxidation (pre-oxidation at 220-250℃ in air for 2-5h), carbonization (carbonization at 2-5℃ / min from room temperature to 800-1000℃ under a nitrogen gas stream with a purity of not less than 99.9% for 2-4h), and reduction treatment (reduction at 550-650℃ for 8-10h under argon-hydrogen mixed gas; the argon-hydrogen mixed gas has a volume ratio of Ar2:H2=(90-95):(5-10)) to obtain a porous Sn-carbon nanofiber composite (porous Sn-CNF composite).

[0007] Step 3: The porous Sn-carbon nanofiber composite was subjected to acid treatment in an acid solution (obtained by mixing 40% HF and 65-68% HNO3 with a volume ratio of 1:1) for 8-12h to obtain the self-supporting porous carbon nanofiber material.

[0008] The above scheme is complicated in terms of steps, and in Step 2, pre-oxidation, carbonization, and reduction (in argon-hydrogen mixed atmosphere) are carried out in multiple steps. The reduction step in argon-hydrogen mixed atmosphere is dangerous if not handled properly, and it is costly and energy-consuming. In addition, the etching process uses extremely corrosive HF (40%), making the preparation difficult and the industrialization of the material difficult to achieve. SUMMARY

[0009] In view of the deficiencies of the prior art, a simple and controllable method for preparing a flexible porous carbon nanofiber membrane electrode is provided. The prepared flexible porous carbon nanofiber membrane electrode has good mechanical properties and can be freely bent and cut. At the same time, it has abundant micropores and mesopores, and this hierarchical porous structure has a high specific surface area, thereby significantly improving the desalination performance of the electrode material.

[0010] To solve the above technical problems, the technical scheme adopted by the present application is a flexible porous carbon nanofiber membrane electrode, taking polyacrylonitrile and a non-metallic oxide used as a porogen as a precursor, through the processes of electrospinning, carbonization and alkaline solution etching, a flexible porous carbon nanofiber membrane electrode with a hierarchical porous structure is obtained, and the mass ratio of polyacrylonitrile to non-metallic oxide is 3:1-10:1.

[0011] The flexible porous carbon nanofiber membrane electrode described above, wherein the non-metallic oxide is SiO2 molecular sieve, the particle size of the SiO2 molecular sieve is 55-115 nm, and the inorganic non-metallic oxide silicon dioxide molecular sieve is used as a hard template to realize controllable preparation of the hierarchical porous carbon nanofiber, in addition, the hierarchical pore structure can be Na + , Cl - , and the like, and the introduction of the silicon dioxide molecular sieve porogen can improve the mechanical flexibility of the porous carbon nanofiber membrane.

[0012] The flexible porous carbon nanofiber membrane electrode described above has a thickness of 0.3-0.5 mm, and the fiber diameter in the flexible porous carbon nanofiber membrane electrode is 300-750 nm.

[0013] The flexible porous carbon nanofiber membrane electrode described above can withstand a maximum tensile stress of 0.56 MPa, and the tensile strain can reach 1.572% at this time.

[0014] The flexible porous carbon nanofiber membrane electrode described above has abundant micropores and mesopores, and the specific surface area is 657.568 m 2 / g.

[0015] The flexible porous carbon nanofiber membrane electrode described above has an optimal desalination amount of 38.64 mg / g in a 600 mg / L sodium chloride solution, a maximum average desalination rate of 12.10 mg / (g x min), and a charge efficiency of 82.36%.

[0016] The flexible porous carbon nanofiber membrane electrode described above has flexibility and can be freely bent and cut.

[0017] The preparation method of the flexible porous carbon nanofiber membrane electrode described above comprises the following steps:

[0018] (1) At room temperature, 20-25 g of tetrapropylammonium hydroxide aqueous solution (25 wt%) was added to 10-14 g of deionized water and stirred for 1-10 min; then, 10-20 g of tetraethyl orthosilicate was added and stirred overnight; the above mixed solution was loaded into a 100 mL reaction kettle, hydrothermal crystallization was carried out at 60-100 °C for 24-48 h, and cooling was carried out for 2-5 h; finally, the product was washed several times with deionized water and ethanol, and dried at 50-80 °C for 10-24 h to obtain SiO2 molecular sieve;

[0019] (2) Polyacrylonitrile (M w = 150000), SiO2 molecular sieve, and N,N-dimethylformamide were weighed in a ratio of 0.4-0.8 g:0.01-0.5 g:3-5 g; first, the SiO2 molecular sieve was uniformly dispersed in N,N-dimethylformamide under the action of ultrasonic waves; then, the polyacrylonitrile powder was dissolved in the mixed solution under continuous stirring at 50-60 °C, and the precursor solution was cooled to room temperature and then transferred into a 5 mL syringe for electrospinning;

[0020] (3) The prepared polyacrylonitrile / SiO2 fiber membrane was heated at a rate of 0.5-2 °C / min to 240-280 °C in an air atmosphere, maintained for 100-150 min; then carbonization was carried out, heated to 700-1000 °C at a rate of 2-10 °C / min under a nitrogen or argon atmosphere, and maintained for 90-120 min, and cooled to room temperature in the furnace to obtain a SiO2-containing carbon nanofiber membrane;

[0021] (4) The obtained SiO2-containing carbon nanofiber membrane was etched with a 3-5 mol / L alkali solution at 60-80 °C for 5-8 h, then washed several times with deionized water until the washing liquid was neutral, and the obtained sample was dried at 60-80 °C for 10-24 h to obtain a flexible porous carbon nanofiber membrane electrode.

[0022] In the above method for preparing the flexible porous carbon nanofiber membrane electrode, in step (2), the syringe needle was connected to the positive electrode clip of a high-voltage power supply, and the electrospinning process parameters were as follows: the tip of the needle was about 16-20 cm away from the drum, the spinning voltage was 10-25 kV, the spinning liquid advancing speed was 0.7-0.9 mL / min, the humidity was controlled at 30-50%, and the temperature was controlled at 20-30 °C.

[0023] The flexible porous carbon nanofiber membrane electrode was applied to seawater desalination, which could reduce the ion concentration in seawater in a short time (30 min), and the conductivity of seawater decreased from 41003 μS / cm to 28983 μS / cm, and the total ion concentration decreased from 26836 mg / L to 17237 mg / L.

[0024] The flexible porous carbon nanofiber membrane electrode, the preparation method and the application have the beneficial effects that the technology is mainly aimed at the low desalination rate of carbon-based electrode materials, and provides a flexible porous carbon nanofiber membrane electrode, a preparation method and an application, so that the specific surface area and mechanical properties of the electrode material are improved, and the electrode material can be better applied to the capacitive deionization field as a self-supporting electrode.

[0025] The polyacrylonitrile / silica molecular sieve precursor nanofiber is prepared by using an electrostatic spinning method, then a carbon / silica nanofiber with a good graphitization degree is obtained through a pre-oxidation and carbonization process, and finally the porous carbon nanofiber is obtained through sodium hydroxide solution etching, cleaning and drying. The porous carbon nanofiber has abundant micropores and mesopores, and the hierarchical porous structure has a high specific surface area, so that the desalination performance of the electrode material is obviously improved. In addition, compared with the etching process in the prior art which uses highly corrosive HF, the sodium hydroxide solution etching is simpler to prepare, and the industrialization of the material is facilitated.

[0026] The method for preparing the flexible porous carbon nanofiber membrane electrode is a simple and controllable preparation method which is easy to popularize, the polyacrylonitrile used has a high carbon yield, and the obtained carbon product has excellent comprehensive performance, and is the most commonly used precursor for preparing carbon fibers. The introduction of the inorganic non-metallic oxide silica molecular sieve pore former realizes the controllable preparation of the hierarchical porous carbon nanofiber, and significantly improves the mechanical flexibility of the porous carbon nanofiber membrane, and the above two aspects provide the possibility for the application of the electrode in a high-performance self-supporting CDI electrode.

[0027] The prepared flexible porous carbon nanofiber membrane electrode has good mechanical properties and can be freely bent and cut; the flexible porous carbon nanofiber membrane electrode is subjected to a tensile test, and the maximum tensile stress that can be borne is 0.56 MPa, and at this time, the tensile strain can reach 1.572%. At the same time, the flexible porous carbon nanofiber membrane electrode has abundant micropores and mesopores, and the hierarchical porous structure has a high specific surface area (657.568 m 2 / g), so that the desalination performance of the electrode material is obviously improved. In a 600 mg / L sodium chloride solution, the optimal desalination amount of the porous carbon nanofiber flexible membrane reaches 38.64 mg / g, the maximum average desalination rate can reach 12.10 mg / (g x min), and the charge efficiency can reach 82.36%, which is significantly higher than other carbon-based nanofiber electrode materials reported at present. The CDI device based on the porous carbon nanofiber flexible membrane can also process actual seawater samples (Yellow Sea), and can adsorb ions in seawater in a short time, further proving the excellent desalination performance. The flexible porous carbon nanofiber membrane electrode material prepared by the application has the advantages of large specific surface area, good flexibility, high desalination rate, long service life and the like.

[0028] Compared with the prior art, the etching process uses HF with extremely strong corrosion, the application uses 3-5 mol / L alkali solution, can completely etch SiO2, and is simpler to prepare, facilitating the industrialization of the material. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 A transmission electron microscope picture of the flexible porous carbon nanofiber membrane electrode;

[0030] Figure 2 A scanning electron microscope picture of the flexible porous carbon nanofiber membrane electrode;

[0031] Figure 3 An X-ray diffraction spectrum of the flexible porous carbon nanofiber membrane electrode;

[0032] Figure 4 A stress-strain curve of the flexible porous carbon nanofiber membrane electrode;

[0033] Figure 5 An optical picture of the folded state of the flexible porous carbon nanofiber membrane electrode;

[0034] Figure 6 An optical picture of the unfolded state of the flexible porous carbon nanofiber membrane electrode;

[0035] Figure 7 A desalination amount comparison chart of the flexible porous carbon nanofiber membrane electrode;

[0036] Figure 8 A desalination chart of the flexible porous carbon nanofiber membrane electrode (PCNFs-3) on actual water samples (Yellow Sea). DETAILED DESCRIPTION

[0037] The application will be described in detail below in combination with the drawings and specific embodiments.

[0038] A flexible porous carbon nanofiber membrane electrode, taking polyacrylonitrile and a non-metallic oxide used as a porogen as a precursor, through electrospinning, carbonization and alkali solution etching process, obtaining a flexible porous carbon nanofiber membrane electrode with hierarchical porous structure, the mass ratio of polyacrylonitrile to non-metallic oxide being 3:1-10:1.

[0039] The flexible porous carbon nanofiber membrane electrode, the non-metallic oxide is SiO2 molecular sieve, the particle size of the SiO2 molecular sieve is 55-115 nm, inorganic non-metallic oxide silica molecular sieve is used as a hard template, controllable preparation of hierarchical porous carbon nanofiber can be realized, in addition, the hierarchical pore structure can be Na + , Cl -The flexible porous carbon nanofiber membrane electrode provides rich electric adsorption sites, thereby improving the ion adsorption capacity of the electrode material. On the other hand, the introduction of the silica molecular sieve pore former can significantly improve the mechanical flexibility of the porous carbon nanofiber membrane, thereby providing an application premise for the self-supporting electrode material. 2

[0040] Example 1

[0041] The preparation method of the flexible porous carbon nanofiber membrane electrode includes the following steps:

[0042] (1) At room temperature, 20 g of tetrapropylammonium hydroxide aqueous solution (25 wt%) is added to 10 g of deionized water and stirred for 1 min. Then, 10 g of tetraethyl orthosilicate is added and stirred overnight; the above-mentioned mixed solution is loaded into a 100 mL reaction kettle, and hydrothermal crystallization is carried out at 60°C for 24 h, and then cooled for 2 h; finally, the product is washed with deionized water and ethanol for several times, and dried at 50°C for 10 h to obtain SiO2 molecular sieve; in the present application, the inorganic non-metallic oxide silica molecular sieve is used as a hard template to prepare a porous carbon nanofiber flexible membrane. The silica molecular sieve has a uniform mesoporous structure, and the size and pore diameter of the molecular sieve particles can be adjusted and controlled, so that the preparation of carbon nanofiber with hierarchical pore structure can be realized. In addition, the hierarchical pore structure can provide Na + , Cl - The flexible porous carbon nanofiber membrane electrode provides rich electric adsorption sites, thereby improving the ion adsorption capacity of the electrode material. On the other hand, the introduction of the silica molecular sieve pore former can significantly improve the mechanical flexibility of the porous carbon nanofiber membrane, thereby providing an application premise for the self-supporting electrode material.

[0043] (2) Polyacrylonitrile (M w =150000), SiO2 molecular sieve, and N,N-dimethylformamide are weighed in a ratio of 0.4 g:0.01 g:3 g; first, the SiO2 molecular sieve is uniformly dispersed in N,N-dimethylformamide under the action of ultrasonic waves; then, the polyacrylonitrile powder is dissolved in the mixed solution under continuous stirring at 50°C; after the precursor solution is cooled to room temperature, it is transferred into a 5 mL syringe for electrospinning;

[0044] (3) The prepared polyacrylonitrile / SiO2 fiber membrane is heated to 240°C at a rate of 0.5°C / min in an air atmosphere, and maintained for 100 min; then, carbonization is carried out, and the temperature is increased to 700°C at a rate of 2°C / min in a nitrogen or argon atmosphere, and maintained for 90 min; the furnace is cooled to room temperature to obtain a carbon nanofiber membrane containing SiO2;

[0045] ​(4), the obtained SiO2-containing carbon nanofiber membrane is etched with 3 mol / L alkali solution at 60°C for 5 h, then washed with deionized water for several times until the washing liquid is neutral, and the obtained sample is dried at 60°C for 10 h to obtain a flexible porous carbon nanofiber membrane electrode. The alkali solution is NaOH solution.

[0046] In the step (2), the syringe needle is connected with the positive electrode clip of the high-voltage power supply, and the electrospinning process parameters are as follows: the distance between the tip of the needle and the drum is about 16 cm, the spinning voltage is 10 kV, the spinning liquid pushing speed is 0.7 mL / min, the humidity is controlled at 30%, and the temperature is controlled at 20°C.

[0047] Example 2

[0048] The same part of this example as example 1 is not described again, and the difference is that the preparation method of the above-mentioned flexible porous carbon nanofiber membrane electrode comprises the following steps:

[0049] (1), at room temperature, 22 g of tetrapropylammonium hydroxide aqueous solution (25 wt%) is added to 12 g of deionized water and stirred for 5 min. Then, 15 g of tetraethyl orthosilicate is added and stirred overnight; the above-mentioned mixed solution is loaded into a 100 mL reaction kettle, hydrothermal crystallization is carried out at 80°C for 35 h, and cooling is carried out for 4 h; finally, the SiO2 molecular sieve is washed with deionized water and ethanol for several times, and dried at 60°C for 18 h to obtain the SiO2 molecular sieve;

[0050] (2), polyacrylonitrile (M w =150000), SiO2 molecular sieve, N,N-dimethylformamide, the ratio is 0.6 g:0.3 g:4 g; first, the SiO2 molecular sieve is uniformly dispersed in N,N-dimethylformamide under the action of ultrasonic waves; then, the polyacrylonitrile powder is dissolved in the mixed solution under the condition of continuous stirring at 55°C, and the precursor solution is cooled to room temperature, and then it is transferred into a 5 mL syringe for electrospinning;

[0051] (3), the prepared polyacrylonitrile / SiO2 fiber membrane is heated to 260°C at a rate of 1.5°C / min in an air atmosphere, and maintained for 130 min; then carbonization is carried out, and the temperature is increased to 800°C at a rate of 8°C / min in a nitrogen or argon atmosphere, and maintained for 100 min, and the furnace is cooled to room temperature to obtain a SiO2-containing carbon nanofiber membrane;

[0052] (4), the obtained SiO2-containing carbon nanofiber membrane is etched with 3 mol / L alkali solution at 60°C for 5 h, then washed with deionized water for several times until the washing liquid is neutral, and the obtained sample is dried at 60°C for 10 h to obtain a flexible porous carbon nanofiber membrane electrode.

[0053] In step (2), the syringe needle is connected to the positive electrode clamp of the high-voltage power supply, and the electrospinning process parameters are as follows: the distance between the needle tip and the drum is about 18 cm, the spinning voltage is 15 kV, the spinning liquid propelling speed is 0.8 mL / min, the humidity is controlled at 40%, and the temperature is controlled at 25°C.

[0054] Example 3

[0055] The same parts of this example as those of Example 1 will not be described again, and the difference lies in that the preparation method of the flexible porous carbon nanofiber membrane electrode comprises the following steps:

[0056] (1) At room temperature, 25 g of tetrapropylammonium hydroxide aqueous solution (25 wt%) is added to 14 g of deionized water and stirred for 10 min. Then, 20 g of tetraethyl orthosilicate is added and stirred overnight; the above-mentioned mixed solution is loaded into a 100 mL reaction kettle, hydrothermal crystallization is carried out at 100°C for 48 h, and cooling is carried out for 5 h; finally, the product is washed with deionized water and ethanol for several times and dried at 80°C for 24 h to obtain SiO2 molecular sieve;

[0057] (2) Polyacrylonitrile (M w =150000), SiO2 molecular sieve, and N,N-dimethylformamide are weighed in a ratio of 0.8 g:0.5 g:5 g; first, the SiO2 molecular sieve is uniformly dispersed in N,N-dimethylformamide under the action of ultrasonic waves; then, the polyacrylonitrile powder is dissolved in the mixed solution under the condition of continuous stirring at 60°C, and the precursor solution is cooled to room temperature and then transferred into a 5 mL syringe for electrospinning;

[0058] (3) The prepared polyacrylonitrile / SiO2 fiber membrane is heated to 280°C at a rate of 2°C / min in an air atmosphere, maintained for 150 min; then, carbonization is carried out, and the temperature is raised to 1000°C at a rate of 10°C / min in a nitrogen or argon atmosphere, and maintained for 120 min, and the furnace is cooled to room temperature to obtain a SiO2-containing carbon nanofiber membrane;

[0059] (4) The obtained SiO2-containing carbon nanofiber membrane is etched with a 5 mol / L alkali solution at 80°C for 8 h, then washed with deionized water several times until the washing liquid is neutral, and the obtained sample is dried at 80°C for 24 h to obtain a flexible porous carbon nanofiber membrane electrode.

[0060] In step (2), the syringe needle is connected to the positive electrode clamp of the high-voltage power supply, and the electrospinning process parameters are as follows: the distance between the needle tip and the drum is about 20 cm, the spinning voltage is 25 kV, the spinning liquid propelling speed is 0.9 mL / min, the humidity is controlled at 50%, and the temperature is controlled at 30°C.

[0061] As Figure 5 ,6 As shown, the flexible porous carbon nanofiber membrane electrode has flexibility and can be freely bent and cut. According to different proportions, they are named PCNFs-1, PCNFs-2, PCNFs-3, respectively. The mechanical properties are as follows: Figure 4 As shown. The elongation at break is 1.572%-1.870%, and the breaking strength is 0.21 MPa-0.56 MPa. The flexible porous carbon nanofiber membrane electrode is subjected to tensile test, and the maximum tensile stress that can be tolerated is 0.56 MPa, and the tensile strain at this time can reach 1.572%.

[0062] Example 4

[0063] The same parts of this example as example 1 are not described again, and the difference is that the flexible porous carbon nanofiber membrane electrode is applied to seawater desalination, which can reduce the ion concentration in seawater in a short time (30 min), and the conductivity of seawater decreases from 41003 μS / cm to 28983 μS / cm, and the total ion concentration decreases from 26836 mg / L to 17237 mg / L.

[0064] In a 600 mg / L sodium chloride solution, the optimal desalination amount of the material reaches 38.64 mg / g, the maximum average desalination rate can reach 12.10 mg / (g×min), and the charge efficiency can reach 82.36%.

[0065] The performance test method of this example includes the following steps:

[0066] (1) Assembly of CDI working battery: the flexible porous carbon nanofiber membrane electrode is used as the self-supporting electrode of CDI, with an area of about 4*4 cm 2 , and a weight of 45-55 mg. The flexible porous carbon nanofiber membrane electrode is directly fixed on the titanium sheet current collector with conductive tape; then, the two CDI electrodes are placed in parallel and separated by a layer of separator to prevent short circuiting of the electrodes; in this experimental example, the amount of NaOH solution used during preparation is 10-20 mL.

[0067] (2) Parameter setting during CDI test: the direct current constant voltage power supply provides a voltage of 1-1.6 V, the concentration of sodium chloride salt solution is 400-600 mg / L, the circulating flow rate of the salt solution is set to 10-30 mL / min, the total volume of the salt solution is 50-100 mL, and the conductivity meter is used to monitor and record the change of the conductivity of the sodium chloride salt solution in real time;

[0068] (3) Calculation of CDI desalination amount (Q, mg / g), desalination rate [q, mg / (g×min)], and charge efficiency (Λ, %): Q = [(C0-C e)] x V / m;

[0069] q = Q / t;

[0070] A = [(C0-C e ) x V x F] / (∫Idt x M x 1000);

[0071] Wherein, C0(mg / L), C e (mg / L), V(L), m(g), t(s), F(C / mol), ∫Idt, M(g / mol) are initial concentration of salt solution, concentration of salt solution at the end of adsorption, volume of salt solution, total mass of capacitive deionization symmetrical electrode, time required for salt solution to reach the end of adsorption, Faraday constant (96485 C / mol), integral of current during adsorption process, molar mass of sodium chloride, respectively.

[0072] The prepared flexible porous carbon nanofiber membrane electrode has good mechanical performance and can be freely bent and cut; at the same time, it has rich micropores and mesopores, and the hierarchical porous structure has a high specific surface area (657.568 m 2 / g), thereby obviously improving the desalination performance of the electrode material. In a 600 mg / L sodium chloride solution, the optimal desalination amount of the flexible porous carbon nanofiber membrane electrode reaches 38.64 mg / g, the maximum average desalination rate can reach 12.10 mg / (g x min), and the charge efficiency can reach 82.36%, which is significantly higher than other carbon-based nanofiber electrode materials reported at present. The CDI device based on the flexible porous carbon nanofiber membrane electrode can also process actual seawater samples (Yellow Sea), and can adsorb ions in seawater in a short time, further proving its excellent desalination performance. The flexible porous carbon nanofiber membrane electrode prepared in the application has the advantages of large specific surface area, good flexibility, high desalination rate, long service life in circulation and the like.

[0073] Of course, the above description is not a limitation of the application, and the application is not limited to the above examples. Changes, modifications, additions or substitutions made by ordinary skilled in the art within the essential scope of the application should also be within the protection scope of the application.

Claims

1. A flexible porous carbon nanofiber membrane electrode, characterized in that: Using polyacrylonitrile and non-metallic oxides as pore-forming agents as precursors, a flexible porous carbon nanofiber membrane electrode with a hierarchical porous structure is obtained through electrospinning, carbonization, and alkaline etching processes. The mass ratio of polyacrylonitrile to non-metallic oxide is 3:1-10:

1. The non-metallic oxide is SiO2 molecular sieve with a particle size of 55-115 nm. Using inorganic non-metallic oxide silica molecular sieve as a hard template enables the controllable preparation of hierarchical porous carbon nanofibers. Furthermore, the hierarchical pore structure can be Na... + Cl - The flexible porous carbon nanofiber membrane electrode provides abundant electroadsorption sites, thereby improving the ion adsorption capacity of the electrode material. Furthermore, the introduction of silica molecular sieve porogens improves the mechanical flexibility of the porous carbon nanofiber membrane. The thickness of the flexible porous carbon nanofiber membrane electrode is 0.3-0.5 mm, and the fiber diameter is 300-750 nm. It can withstand a maximum tensile stress of 0.56 MPa, at which point the tensile strain reaches 1.572%. In a 600 mg / L sodium chloride solution, the optimal desalination capacity of this material reaches 38.64 mg / g, the maximum average desalination rate reaches 12.10 mg / (g×min), and the charge efficiency reaches 82.36%. The flexible porous carbon nanofiber membrane electrode possesses abundant micropores and mesopores, with a specific surface area of ​​657.568 m². 2 / g; The flexible porous carbon nanofiber membrane electrode is flexible and can be freely bent and cut.

2. The method for preparing the flexible porous carbon nanofiber membrane electrode according to claim 1, characterized in that: Includes the following steps: (1) At room temperature, add 20-25g of 25wt% tetrapropylammonium hydroxide aqueous solution to 10-14g of deionized water and stir for 1-10min; then add 10-20g of tetraethyl orthosilicate and stir overnight; put the above mixed solution into a 100mL reactor and hydrothermally crystallize at 60-100℃ for 24-48h, then cool for 2-5h; finally, wash several times with deionized water and ethanol, and dry at 50-80℃ for 10-24h to obtain SiO2 molecular sieve; (2) Weigh M w =150000 polyacrylonitrile, SiO2 molecular sieve, N,N-dimethylformamide, in a ratio of 0.4-0.8g:0.01-0.5g:3-5g; First, under ultrasonic treatment, the SiO2 molecular sieve is evenly dispersed in N,N-dimethylformamide; then, under continuous stirring at 50-60℃, the polyacrylonitrile powder is dissolved in the mixture. After the precursor solution cools to room temperature, it is transferred into a 5mL syringe for electrospinning. (3) The obtained polyacrylonitrile / SiO2 fiber membrane is heated to 240-280℃ at a rate of 0.5-2℃ / min in air atmosphere and held for 100-150min; then carbonization is carried out by heating to 700-1000℃ at a rate of 2-10℃ / min in nitrogen or argon atmosphere and holding for 90-120min, and then cooling to room temperature in furnace to obtain a carbon nanofiber membrane containing SiO2. (4) The obtained carbon nanofiber membrane containing SiO2 is etched with 3-5 mol / L alkaline solution at 60-80℃ for 5-8h, and then washed with deionized water several times until the washing solution is neutral. The obtained sample is dried at 60-80℃ for 10-24h to obtain a flexible porous carbon nanofiber membrane electrode. In step (2), the syringe needle is connected to the positive terminal clamp of the high voltage power supply. The electrospinning process parameters are: the needle tip is 16-20cm away from the roller, the spinning voltage is 10-25kV, the spinning solution propulsion speed is 0.7-0.9mL / min, the humidity is controlled at 30-50%, and the temperature is controlled at 20-30℃.

3. The application of the flexible porous carbon nanofiber membrane electrode according to claim 1, characterized in that: When applied to seawater desalination, it can reduce the ion concentration in seawater within a short period of 30 minutes. The conductivity of seawater decreased from 41003 μS / cm to 28983 μS / cm, and the total ion concentration decreased from 26836 mg / L to 17237 mg / L.

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