CN114899463B - Organic-inorganic composite medium-temperature proton exchange membrane and preparation method thereof - Google Patents

Abstract

The present invention relates to the field of fuel cell technology, and specifically discloses an organic-inorganic composite medium-temperature proton exchange membrane and a preparation method thereof. The organic-inorganic composite medium-temperature proton exchange membrane comprises chitosan and hollow silicon dioxide nanotubes, wherein the hollow silicon dioxide nanotubes are dispersed in a chitosan matrix, and the addition amount thereof is 0.5wt.%-30wt.% of chitosan. The preparation method thereof comprises the following steps: 1) oxidizing carbon nanotubes; 2) preparing silicon dioxide-coated carbon nanotubes by a sol-gel method; 3) calcining silicon dioxide-coated carbon nanotubes at high temperature to obtain hollow silicon dioxide nanotubes; 4) uniformly mixing a dispersion of the hollow silicon dioxide nanotubes with a solution of chitosan, and performing solution casting to form a film; 5) cross-linking the dry film with a cross-linking agent to obtain an organic-inorganic composite medium-temperature proton exchange membrane. The obtained composite membrane still has high proton conductivity and mechanical strength under high temperature and anhydrous conditions.

Description

An organic-inorganic composite medium-temperature proton exchange membrane and a preparation method thereof

Technical Field

The present invention relates to the technical field of fuel cells, and in particular to an organic-inorganic composite medium-temperature proton exchange membrane and a preparation method thereof.

Background technique

The proton exchange membrane is an absolutely critical component of the proton exchange membrane fuel cell. It mainly plays the role of transmitting protons, isolating electrons, and preventing the oxygen and fuel from penetrating and contacting the positive and negative levels. Excellent proton exchange membranes are required to have high proton transmission capacity, and also need to have good proton conductivity and certain mechanical strength under high temperature and low humidity working conditions. The series of membranes have high proton conductivity under saturated humidity, but at high temperature (>100°C) the proton conductivity drops sharply due to water loss in the membrane, making it difficult to use in medium and high temperature proton exchange membrane fuel cells. In addition, the preparation process of the series of membranes is extremely complicated and the price is very expensive. Therefore, it is very important to find a proton exchange membrane that has high proton conductivity and mechanical strength under high temperature and low humidity conditions and is inexpensive.

Chitosan is widely found in shrimp shells and crab shells. It is the product of chitosan deacetylation. It has a wide source, low price, good film-forming properties, and a simple and environmentally friendly film-forming process. In addition, the ring structure on the chitosan molecule ensures the thermochemical stability of the chitosan film and sufficient mechanical properties. The presence of hydroxyl and amino groups on the chitosan monomer indicates that the chitosan membrane has the potential to be prepared into a high-performance proton exchange membrane. However, studies have shown that the proton conductivity of the chitosan dry membrane without cross-linking and modification at room temperature is only ^10-9 S/cm ^-1 , which is equivalent to an insulating material, and its mechanical strength also needs to be further improved. Therefore, it is necessary to modify the chitosan to improve its proton conductivity and mechanical strength.

In view of this, the present invention is proposed.

Summary of the invention

In view of the deficiencies in the prior art, the present invention aims to provide an organic-inorganic composite medium-temperature proton exchange membrane and a preparation method thereof.

In order to achieve the above-mentioned purpose of the invention, the present invention provides an organic-inorganic composite medium-temperature proton exchange membrane, which is cast into a membrane by a solution including chitosan and hollow silica nanotubes, wherein the hollow silica nanotubes are dispersed in a chitosan matrix, and the addition amount thereof is 0.5wt.%-30wt.% of the chitosan.

Furthermore, the preparation method of the hollow silica nanotubes is as follows: using tetraethyl orthosilicate as a raw material and oxidized (or acidified) carbon nanotubes as a template, coating silica on the surface of the oxidized (or acidified) carbon nanotube template by a sol-gel method to obtain silica-coated carbon nanotubes; then calcining the silica-coated carbon nanotubes at a high temperature of 700-1000° C. in an air atmosphere to remove the carbon nanotube template, thereby obtaining hollow silica nanotubes.

Furthermore, the ratio of tetraethyl orthosilicate to oxidized (or acidified) carbon nanotubes is 5 mL/1 g to 50 mL/1 g.

Furthermore, the preparation method of the organic-inorganic composite medium-temperature proton exchange membrane comprises the following steps: 1) oxidizing carbon nanotubes; 2) preparing silicon dioxide-coated carbon nanotubes by a sol-gel method; 3) calcining the silicon dioxide-coated carbon nanotubes at high temperature to obtain silicon dioxide hollow nanotubes; 4) uniformly mixing the dispersion of the silicon dioxide hollow nanotubes with a chitosan solution, and performing solution casting to form a membrane; 5) cross-linking the dry membrane with a cross-linking agent to obtain an organic-inorganic composite medium-temperature proton exchange membrane.

The method for preparing the organic-inorganic composite proton exchange membrane provided by the present invention specifically comprises the following steps:

(1) Preparation of oxidized carbon nanotubes: 1 g of carbon nanotubes is added to 50-150 ml of a strong oxidizing solution and oxidized at room temperature to 120° C. for 1-12 h, then cooled to room temperature, filtered, repeatedly washed with deionized water until the filtrate is neutral, and dried to obtain oxidized carbon nanotubes;

(2) dispersing the oxidized carbon nanotubes obtained in step (1) in a mixed solution of water and a co-solvent, adding ammonia water to adjust the pH value of the solution to 8-10, and ultrasonically oscillating at room temperature to obtain a uniformly dispersed oxidized carbon nanotube dispersion;

(3) Preparation of silicon dioxide-coated carbon nanotubes: Tetraethyl orthosilicate is slowly added dropwise to the oxidized carbon nanotube dispersion obtained in step (2) under stirring, and the ratio of tetraethyl orthosilicate to oxidized carbon nanotubes is controlled to be 5 mL/1 g to 50 mL/1 g. After the addition is completed, the reaction is continued at room temperature to 60° C. with magnetic stirring for 0.5 to 12 hours. After the reaction is completed, the carbon nanotubes coated with silicon dioxide are obtained by suction filtration, washing, and vacuum drying for 8 to 24 hours;

(4) calcining the silicon dioxide-coated carbon nanotubes obtained in step (3) in an air atmosphere at 700-1000° C. for 1-6 hours to obtain silicon dioxide hollow nanotubes;

(5) dispersing the hollow silica nanotubes obtained in step (4) in water or ethanol to prepare a dispersion having a concentration of 1 g/10 mL to 1 g/25 mL;

(6) dissolving chitosan in a 1-3 vol.% acetic acid aqueous solution to prepare a chitosan solution with a concentration of 0.5 wt.% to 5 wt.%, adding all of the hollow silica nanotube dispersion prepared in step (5) to the chitosan solution, and mixing thoroughly to obtain a uniform dispersion;

(7) casting the uniform dispersion obtained in step (6) onto a clean glass plate to form a film, drying it, cooling it to room temperature, and removing the film;

(8) Soaking the dry membrane obtained in step (7) in a sulfuric acid solution with a concentration of 0.1 to 4 mol/L at room temperature for 0.5 to 5 hours to cross-link the composite membrane, and repeatedly rinsing the cross-linked membrane with deionized water and then drying to obtain the organic-inorganic composite proton exchange membrane.

Furthermore, the strong oxidizing solution in step (1) is 98 wt % or more of sulfuric acid or 65 wt % or more of nitric acid, or a mixture of the two in any volume ratio;

Furthermore, the co-solvent in step (2) is any one of methanol, ethanol, n-propanol, isopropanol, n-butanol, or a mixture of two thereof; the ratio of carbon nanotubes to co-solvent is 1 g/50 mL to 1 g/500 mL, and the volume ratio of co-solvent to water is 0.5:1 to 5:1; the concentration of aqueous ammonia is 20 wt% to 30 wt%, preferably 25 wt%;

Furthermore, the amount of hollow silica nanotubes added in step (6) is 0.5wt.%-30wt.% of the mass of chitosan; preferably, the amount of hollow silica nanotubes added is 6.6wt.%-30wt.% of the mass of chitosan; further preferably, the amount of hollow silica nanotubes added is 10wt.%-30wt.% of the mass of chitosan; more preferably, the amount of hollow silica nanotubes added is 10wt.%-20wt.% of the mass of chitosan.

The organic-inorganic composite intermediate-temperature proton exchange membrane provided by the present invention or the organic-inorganic composite intermediate-temperature proton exchange membrane prepared by the above method is used in the preparation of a proton exchange membrane fuel cell.

In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects compared with the prior art:

1. The organic-inorganic composite medium-temperature proton exchange membrane provided by the present invention has low density, high mechanical strength, and is evenly dispersed in the chitosan matrix. Therefore, a small amount of hollow silica nanotubes can greatly improve the mechanical properties of the composite membrane;

2. The organic-inorganic composite medium-temperature proton exchange membrane provided by the present invention has better compatibility with the hydrophilic chitosan membrane matrix due to the strong hydrophilicity of the hollow silica nanotubes, which can greatly improve the dispersion of the nanotubes in the polymer matrix (see Figures 3 and 4), thereby further improving the mechanical properties of the composite membrane (the tensile strength of the composite membrane is increased by 33.8% to 69.4% compared with the pure chitosan membrane; under the same addition amount, the tensile strength of the composite membrane is increased by 96.8% compared with the chitosan/carbon nanotube composite membrane);

3. Compared with the carbon nanotube composite membrane coated with silica, the organic-inorganic composite medium-temperature proton exchange membrane provided by the present invention has a hydrophilic hollow structure of the hollow silica nanotubes that can act as a "water reservoir" in the composite system, thereby effectively delaying the evaporation and loss of water in the membrane. In addition, the hydrophilic outer wall and hollow inner cavity of the tube can form more hydrophilic channels. These hydrophilic hollow tubes can retain water or provide additional proton migration channels in the composite membrane. Therefore, it can also obtain a higher proton conductivity under high temperature and low humidity conditions (the anhydrous proton conductivity is increased by 53% compared with the carbon nanotube composite membrane coated with the same amount of silica added).

In summary, the organic-inorganic medium-temperature composite proton exchange membrane prepared by chitosan and silica hollow nanotubes is expected to have broad application prospects in medium-temperature proton exchange membrane fuel cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transmission electron microscope image of the silicon dioxide-coated carbon nanotubes prepared in Example 1.

FIG. 2 is a transmission electron microscope image of the hollow silicon dioxide nanotubes prepared in Example 1.

FIG3 is a cross-sectional SEM image of the chitosan/silicon dioxide hollow nanotube organic-inorganic composite intermediate temperature proton exchange membrane prepared in Example 1.

FIG. 4 is a cross-sectional SEM image of the chitosan/carbon nanotube composite film prepared in Comparative Example 2.

Detailed ways

The applicant will further explain the technical solution of the present invention in detail below in conjunction with specific embodiments, with the aim of enabling those skilled in the art to have a clearer understanding and recognition of the present application.

The following specific embodiments should not be understood or interpreted in any degree as limiting the scope of protection requested by the claims of this application.

In the following Examples 1-5 and Comparative Examples:

The raw carbon nanotubes used were all multi-walled carbon nanotubes with a diameter of 15-40 nm and a length of 5-15 μm, purchased from Shenzhen Nanoport Co., Ltd.; chitosan had a molecular weight of 1 million and a deacetylation degree of 90%, purchased from Zhejiang Aoxing Biological Co., Ltd.

Embodiment 1: A method for preparing an organic-inorganic composite medium-temperature proton exchange membrane, comprising the following steps:

2 g of carbon nanotubes were added to 200 mL of a strong oxidizing solution of 98 wt% concentrated sulfuric acid and 65 wt% concentrated nitric acid (volume ratio 1:3) for oxidation treatment at 100 °C for 4 h, then cooled to room temperature, filtered, repeatedly washed with deionized water until the filtrate was neutral, and dried to obtain oxidized carbon nanotubes;

1 g of oxidized carbon nanotubes was dispersed in a mixed solution of 50 mL of water and 150 mL of anhydrous ethanol, 25 wt % of ammonia water was added dropwise to adjust the pH value of the mixed solution to 9, and ultrasonic oscillation was performed at room temperature to obtain a uniformly dispersed oxidized carbon nanotube dispersion; 20 mL of tetraethyl orthosilicate was slowly added dropwise to the oxidized carbon nanotube dispersion under stirring, and after the addition was completed, the reaction was continued under magnetic stirring at room temperature for 12 hours. After the reaction was completed, the carbon nanotubes coated with silicon dioxide were obtained by suction filtration, washing, and vacuum drying for 12 hours; the transmission electron microscope image thereof is shown in FIG1;

The dried silica-coated carbon nanotubes were placed in a muffle furnace, heated to 800°C at a heating rate of 20°C/min in an air atmosphere, and calcined for 4 hours to obtain hollow silica nanotubes; the transmission electron microscope image thereof is shown in FIG2 ; 1 g of hollow silica nanotubes was dispersed in 15 mL of anhydrous ethanol to prepare a dispersion;

15g of chitosan was dissolved in 1vol.% acetic acid aqueous solution to prepare a chitosan solution with a concentration of 2wt%. The hollow silica nanotube dispersion was completely added to the chitosan solution (wherein: the mass of the hollow silica nanotubes was 6.6wt% of the mass of the chitosan). After sufficient mixing, a uniform dispersion was obtained. The dispersion was cast into a film on a clean glass plate, dried, and cooled to room temperature for demolding. Finally, the obtained dry film was immersed in a 1mol/L sulfuric acid solution at room temperature for 3h to cross-link the composite membrane. The cross-linked membrane was repeatedly rinsed with deionized water and dried to obtain the organic-inorganic composite medium-temperature proton exchange membrane, and its cross-sectional SEM image is shown in Figure 3.

Comparative Example 1: Preparation of pure chitosan film, the steps are:

15g of chitosan was dissolved in 1vol.% acetic acid aqueous solution to prepare a chitosan solution with a concentration of 2wt%. The chitosan solution was cast into a film on a clean glass plate, dried, and cooled to room temperature for demolding. Finally, the obtained dry film was immersed in a 1mol/L sulfuric acid solution at room temperature for 3h to cross-link the membrane. The cross-linked membrane was repeatedly rinsed with deionized water and dried to obtain a pure chitosan membrane.

Comparative Example 2: Preparation of chitosan/carbon nanotube composite film, the process is as follows:

1 g of carbon nanotubes was dispersed in 15 mL of anhydrous ethanol to prepare a dispersion; 15 g of chitosan was dissolved in 1 vol.% acetic acid aqueous solution to prepare a chitosan solution with a concentration of 2 wt%, all of the carbon nanotube dispersion was added to the chitosan solution, and a uniform dispersion was obtained after sufficient mixing. The dispersion was cast into a film on a clean glass plate, dried, and cooled to room temperature for film removal; finally, the obtained dry film was immersed in a sulfuric acid solution with a concentration of 1 mol/L at room temperature for 3 hours to cross-link the composite film, and the cross-linked film was repeatedly rinsed with deionized water and dried to obtain a chitosan/carbon nanotube composite film, whose cross-sectional SEM image is shown in Figure 4.

Comparative Example 3: Preparation of chitosan/silicon dioxide coated carbon nanotube composite film, the process is as follows:

2 g of carbon nanotubes were added to 200 mL of a strong oxidizing solution of 98 wt% concentrated sulfuric acid and 65 wt% concentrated nitric acid (volume ratio 1:3) for oxidation treatment at 100 °C for 4 h, then cooled to room temperature, filtered, repeatedly washed with deionized water until the filtrate was neutral, and dried to obtain oxidized carbon nanotubes;

1 g of oxidized carbon nanotubes was dispersed in a mixed solution of 50 mL of water and 150 mL of anhydrous ethanol, 25 wt % of ammonia water was added dropwise to adjust the pH value of the mixed solution to 9, and ultrasonic oscillation was performed at room temperature to obtain a uniformly dispersed oxidized carbon nanotube dispersion; 20 mL of tetraethyl orthosilicate was slowly added dropwise to the oxidized carbon nanotube dispersion under stirring, and after the addition was completed, the reaction was continued under magnetic stirring at room temperature for 12 hours. After the reaction was completed, the carbon nanotubes coated with silicon dioxide were obtained by suction filtration, washing, and vacuum drying for 12 hours;

1g of silicon dioxide-coated carbon nanotubes was dispersed in 15mL of anhydrous ethanol to prepare a dispersion; 15g of chitosan was dissolved in 1vol.% of acetic acid aqueous solution to prepare a chitosan solution with a concentration of 2wt%, all of the dispersion of silicon dioxide-coated carbon nanotubes was added to the chitosan solution, and a uniform dispersion was obtained after sufficient mixing, and the dispersion was cast into a film on a clean glass plate, dried, and cooled to room temperature for demolding; finally, the obtained dry film was immersed in a sulfuric acid solution with a concentration of 1mol/L at room temperature for 3h to cross-link the composite film, and the cross-linked film was repeatedly rinsed with deionized water and dried to obtain a chitosan/silicon dioxide-coated carbon nanotube composite film.

The performance test results of the membranes prepared in the above-mentioned Example 1 and Comparative Examples 1-3 are shown in Table 1.

Table 1

Performance Anhydrous proton conductivity (mS/cm) Tensile strength(MPa) Example 1 16.5 37.2 Comparative Example 1 6.4 27.5 Comparative Example 2 4.3 18.9 Comparative Example 3 10.8 35.6

Membrane performance test method:

(1) Tensile strength: The film sample was cut into rectangular strips of length × width = 60 × 10 mm and subjected to a tensile test on a Shimadzu AG-IC universal tensile machine at room temperature with a tensile rate of 2 mm/min. The maximum tensile stress to which the film sample was subjected until it broke was recorded as the tensile strength.

(2) Anhydrous proton conductivity: The anhydrous proton conductivity of the membrane was tested in an oven at 120°C without humidification using the AC impedance method on a Swiss Metrohm Autolab 302N frequency response analyzer. The frequency sweep range was 1 to 10 ⁷ Hz and the AC signal amplitude was 100 mV. The cut membrane (4 cm × 5 cm) was placed on a homemade test platform. The proton conductivity σ (S/cm) of the membrane was calculated using the following formula:

Where L and A are the distance between the two electrodes and the effective cross-sectional area of the membrane to be measured between the two electrodes, respectively, and R is the impedance of the membrane, which is obtained through the Nyquist plot.

As can be seen from Figure 1, the outer wall of the carbon nanotube has a relatively uniform light-colored protruding silica coating layer. The silica coating forms a relatively uniform coating layer on the surface of the carbon nanotube, and the coating layer thickness is about 25nm; as can be seen from Figure 2, after calcination, the carbon nanotube has been calcined and removed, forming a hollow silica nanotube with uniform wall thickness, the wall thickness is about 25nm, and the hollow tube cavity diameter is about 20nm. As can be seen from Figure 3, the hollow silica nanotubes are dispersed very uniformly in the composite film. As can be seen from Figure 4, the carbon nanotubes are extremely unevenly dispersed in the composite film, and there are many obvious aggregates (circled in Figure 4).

From the results in Table 1, it can be seen that the anhydrous proton conductivity of the pure chitosan membrane prepared in Comparative Example 1 is 6.4 mS/cm, and the tensile strength is 27.5 MPa; the anhydrous proton conductivity of the chitosan membrane/carbon nanotube composite membrane prepared in Comparative Example 2 is only 4.3 mS/cm, and the tensile strength is only 18.9 MPa. Its proton conductivity and tensile strength are lower than those of the pure chitosan membrane. This is mainly because the hydrophobic structure of the carbon nanotubes themselves greatly reduces the water retention effect of the composite membrane under high temperature and low humidity, and there is a strong van der Waals force between the carbon nanotubes, which makes it difficult to retain water in the chitosan membrane when the added amount is only 6.6%. The chitosan film/silicon dioxide coated carbon nanotube composite film prepared in Comparative Example 3 has an anhydrous proton conductivity of 10.8 mS/cm and a tensile strength of 35.6 MPa, which are improved compared with Comparative Examples 1 and 2. This is mainly because the hydrophilic silica coating layer promotes the dispersion of the carbon nanotubes, which can exert its enhancement effect, and can also appropriately increase the water retention performance of the composite system under high temperature and low humidity, which is beneficial to the improvement of its proton conductivity. The anhydrous proton conductivity of the organic-inorganic composite medium-temperature proton exchange membrane prepared in this embodiment 1 is 16.5mS/cm, which is nearly 2.6 times that of the pure chitosan membrane, and the anhydrous proton conductivity is also increased by 53% compared with that of the comparative example 3. This is mainly because the hollow structure of the hollow silica nanotubes can play the role of "water reservoir" in the composite system, effectively delaying the evaporation and loss of water in the membrane, and its hydrophilic outer wall and hollow inner cavity form more hydrophilic channels. These hydrophilic hollow tubes play the role of water retention or providing additional proton migration channels in the composite membrane, so it can also obtain higher proton conductivity under high temperature and low humidity conditions, and its mechanical strength is also significantly increased. This composite membrane is expected to be used in medium-temperature proton exchange membrane fuel cells.

Embodiment 2: A method for preparing an organic-inorganic composite medium-temperature proton exchange membrane, comprising the following steps:

2 g of carbon nanotubes were added to 100 mL of 98 wt % concentrated sulfuric acid and oxidized at 80° C. for 8 h, then cooled to room temperature, filtered, repeatedly washed with deionized water until the filtrate was neutral, and dried to obtain oxidized carbon nanotubes;

1 g of oxidized carbon nanotubes was dispersed in a mixed solution of 100 mL of water and 50 mL of methanol, 25 wt % of ammonia water was added dropwise to adjust the pH value of the mixed solution to 10, and ultrasonic oscillation was performed at room temperature to obtain a uniformly dispersed oxidized carbon nanotube dispersion; 5 mL of tetraethyl orthosilicate was slowly added dropwise to the oxidized carbon nanotube dispersion under stirring, and after the addition was completed, the temperature was adjusted to 60° C. and magnetic stirring was performed to react for 0.5 hours. After the reaction was completed, the carbon nanotubes coated with silicon dioxide were obtained after filtration, washing, and vacuum drying for 8 hours;

The dried silica-coated carbon nanotubes were placed in a muffle furnace, heated to 1000°C at a heating rate of 20°C/min in an air atmosphere, and calcined for 1 hour to obtain hollow silica nanotubes; 1 g of hollow silica nanotubes was dispersed in 10 mL of water to prepare a dispersion;

3.3 g of chitosan is dissolved in a 3 vol.% acetic acid aqueous solution to prepare a chitosan solution with a concentration of 0.5 wt%. The hollow silica nanotube dispersion is completely added to the chitosan solution (wherein: the mass of the hollow silica nanotubes is 30 wt% of the mass of the chitosan), and a uniform dispersion is obtained after sufficient mixing. The dispersion is cast into a film on a clean glass plate, dried, and cooled to room temperature for demolding. Finally, the obtained dry film is immersed in a sulfuric acid solution with a concentration of 0.1 mol/L at room temperature for 5 hours to cross-link the composite membrane. The cross-linked membrane is repeatedly rinsed with deionized water and dried to obtain the organic-inorganic composite medium-temperature proton exchange membrane.

Embodiment 3: A method for preparing an organic-inorganic composite medium-temperature proton exchange membrane, comprising the following steps:

2 g of carbon nanotubes were added to 300 mL of 65 wt% concentrated nitric acid and oxidized at 120° C. for 1 h, then cooled to room temperature, filtered, repeatedly washed with deionized water until the filtrate was neutral, and dried to obtain oxidized carbon nanotubes;

1 g of oxidized carbon nanotubes was dispersed in a mixed solution of 100 mL of water and 500 mL of n-propanol, 25 wt % of ammonia water was added dropwise to adjust the pH value of the mixed solution to 8, and ultrasonic oscillation was performed at room temperature to obtain a uniformly dispersed oxidized carbon nanotube dispersion; 50 mL of tetraethyl orthosilicate was slowly added dropwise to the oxidized carbon nanotube dispersion under stirring, and after the addition was completed, the temperature was adjusted to 40° C. and magnetic stirring was performed to react for 6 hours. After the reaction was completed, the carbon nanotubes coated with silicon dioxide were obtained by suction filtration, washing, and vacuum drying for 24 hours;

The dried silica-coated carbon nanotubes were placed in a muffle furnace, heated to 700°C at a heating rate of 20°C/min in an air atmosphere, and calcined for 6 hours to obtain hollow silica nanotubes; 1 g of hollow silica nanotubes was dispersed in 25 mL of water to prepare a dispersion;

200g of chitosan is dissolved in a 2vol.% acetic acid aqueous solution to prepare a chitosan solution with a concentration of 5wt%, and all the silica hollow nanotube dispersion is added to the chitosan solution (wherein: the mass of the silica hollow nanotube is 0.5wt% of the mass of the chitosan), and a uniform dispersion is obtained after sufficient mixing. The dispersion is cast into a film on a clean glass plate, dried, and cooled to room temperature for demolding; finally, the obtained dry film is immersed in a sulfuric acid solution with a concentration of 4mol/L at room temperature for 0.5h to cross-link the composite membrane, and the cross-linked membrane is repeatedly rinsed with deionized water and dried to obtain the organic-inorganic composite medium-temperature proton exchange membrane.

Embodiment 4: A method for preparing an organic-inorganic composite medium-temperature proton exchange membrane comprises the following steps:

2 g of carbon nanotubes were added to 240 mL of a strong oxidizing solution of 98 wt% concentrated sulfuric acid and 65 wt% concentrated nitric acid (volume ratio 3:1) and oxidized at room temperature for 12 h, then cooled to room temperature, filtered, repeatedly washed with deionized water until the filtrate was neutral, and dried to obtain oxidized carbon nanotubes;

1 g of oxidized carbon nanotubes was dispersed in a mixed solution of 200 mL of water and 200 mL of isopropanol, 25 wt % of ammonia water was added dropwise to adjust the pH value of the mixed solution to 8.5, and ultrasonic oscillation was performed at room temperature to obtain a uniformly dispersed oxidized carbon nanotube dispersion; 30 mL of tetraethyl orthosilicate was slowly added dropwise to the oxidized carbon nanotube dispersion under stirring, and after the addition was completed, the temperature was adjusted to 50° C. and magnetic stirring was performed for 8 hours. After the reaction was completed, the carbon nanotubes coated with silicon dioxide were obtained by suction filtration, washing, and vacuum drying for 20 hours;

The dried silica-coated carbon nanotubes were placed in a muffle furnace, heated to 900°C at a heating rate of 20°C/min in an air atmosphere, and calcined for 2 hours to obtain hollow silica nanotubes; 1 g of the hollow silica nanotubes was dispersed in 20 mL of anhydrous ethanol to prepare a dispersion;

10g of chitosan is dissolved in a 2.5vol.% acetic acid aqueous solution to prepare a chitosan solution with a concentration of 3wt%, and all the silica hollow nanotube dispersion is added to the chitosan solution (wherein: the mass of the silica hollow nanotube is 10wt% of the mass of the chitosan), and a uniform dispersion is obtained after sufficient mixing. The dispersion is cast into a film on a clean glass plate, dried, and cooled to room temperature for demolding; finally, the obtained dry film is immersed in a 2mol/L sulfuric acid solution at room temperature for 2h to cross-link the composite membrane, and the cross-linked membrane is repeatedly rinsed with deionized water and dried to obtain the organic-inorganic composite medium-temperature proton exchange membrane.

Example 5: A method for preparing an organic-inorganic composite medium-temperature proton exchange membrane, comprising the following steps:

2 g of carbon nanotubes were added to 160 mL of a strong oxidizing solution of 98 wt% concentrated sulfuric acid and 65 wt% concentrated nitric acid (volume ratio 3:1) and oxidized at 60°C for 10 h, then cooled to room temperature, filtered, repeatedly washed with deionized water until the filtrate was neutral, and dried to obtain oxidized carbon nanotubes;

1 g of oxidized carbon nanotubes was dispersed in a mixed solution of 150 mL of water and 300 mL of n-butanol, 25 wt % of ammonia water was added dropwise to adjust the pH value of the mixed solution to 9, and ultrasonic oscillation was performed at room temperature to obtain a uniformly dispersed oxidized carbon nanotube dispersion; 40 mL of tetraethyl orthosilicate was slowly added dropwise to the oxidized carbon nanotube dispersion under stirring, and after the addition was completed, the reaction was continued under magnetic stirring at room temperature for 10 hours. After the reaction was completed, the carbon nanotubes coated with silicon dioxide were obtained by suction filtration, washing, and vacuum drying for 12 hours;

The dried silica-coated carbon nanotubes were placed in a muffle furnace, heated to 850°C at a heating rate of 20°C/min in an air atmosphere, and calcined for 3 hours to obtain hollow silica nanotubes; 1 g of hollow silica nanotubes was dispersed in 18 mL of anhydrous ethanol to prepare a dispersion;

5g of chitosan is dissolved in 1vol.% acetic acid aqueous solution to prepare a chitosan solution with a concentration of 4wt%, and all the silica hollow nanotube dispersion is added to the chitosan solution (wherein: the mass of the silica hollow nanotube is 20wt% of the mass of the chitosan), and a uniform dispersion is obtained after sufficient mixing. The dispersion is cast into a film on a clean glass plate, dried, and cooled to room temperature for demolding; finally, the obtained dry film is immersed in a sulfuric acid solution with a concentration of 3mol/L at room temperature for 1.5h to cross-link the composite membrane, and the cross-linked membrane is repeatedly rinsed with deionized water and dried to obtain the organic-inorganic composite medium-temperature proton exchange membrane.

Table 2 below lists various performance index data of the organic-inorganic composite medium-temperature proton exchange membranes prepared in Examples 1-5.

Table 2

It can be seen from Table 2 that the anhydrous proton conductivity and tensile strength of the composite membranes prepared in Examples 1-5 are better than those of pure chitosan membranes, wherein the anhydrous proton conductivity is increased by 1.1 to 3.1 times that of the pure chitosan membrane, while the tensile strength is increased by 33.8% to 69.4% that of the pure chitosan membrane, and the anhydrous proton conductivity and tensile strength are also better than those of Comparative Examples 2 and 3.

The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principle of the present invention shall be regarded as equivalent replacement methods and are included in the protection scope of the present invention.

Claims (11)

1. The organic-inorganic composite medium temperature proton exchange membrane is characterized by comprising chitosan and silica hollow nanotubes, wherein the silica hollow nanotubes are dispersed in a chitosan matrix, and the addition amount of the silica hollow nanotubes is 0.5-30 wt.% of chitosan;

The preparation method of the organic-inorganic composite medium-temperature proton exchange membrane comprises the following steps: 1) Oxidizing the carbon nano tube; 2) Preparing a carbon nano tube coated with silicon dioxide by a sol-gel method; 3) Calcining the carbon nanotube coated with the silicon dioxide at a high temperature to obtain a silicon dioxide hollow nanotube; 4) Uniformly mixing the dispersion liquid of the silica hollow nanotubes with the solution of chitosan, and performing solution casting to form a film; 5) Crosslinking treatment is carried out on the dry film by using a crosslinking agent, so as to obtain an organic-inorganic composite medium-temperature proton exchange membrane;

The preparation method of the silica hollow nanotube comprises the following steps: using tetraethyl orthosilicate as a raw material, using an oxidized carbon nanotube as a template, and coating silicon dioxide on the surface of the oxidized carbon nanotube template by a sol-gel method to obtain a silicon dioxide coated carbon nanotube; and then calcining the carbon nano tube coated with the silicon dioxide at a high temperature in an air atmosphere at 700-1000 ℃ to remove the carbon nano tube template, thus obtaining the silicon dioxide hollow nano tube.

2. The organic-inorganic composite medium temperature proton exchange membrane according to claim 1, wherein the silica hollow nanotubes are added in an amount of 6.6wt.% to 30wt.% of chitosan.

3. The organic-inorganic composite medium temperature proton exchange membrane according to claim 2, wherein the silica hollow nanotubes are added in an amount of 10wt.% to 30wt.% of chitosan.

4. An organic-inorganic composite medium temperature proton exchange membrane according to claim 3, wherein the silica hollow nanotubes are added in an amount of 10wt.% to 20wt.% of chitosan.

5. The organic-inorganic composite medium temperature proton exchange membrane according to claim 1, wherein the ratio of tetraethyl orthosilicate to carbon oxide nanotubes is 5mL/1 g-50 mL/1g.

6. The method for preparing the organic-inorganic composite medium-temperature proton exchange membrane according to any one of claims 1 to 5, which is characterized by comprising the following steps:

(1) Preparing carbon oxide nanotubes: adding each 1g of carbon nano tube into each 50-150 ml of strong oxidizing solution, oxidizing at room temperature to 120 ℃ for 1-12 h, cooling to room temperature, filtering, repeatedly washing with deionized water until filtrate is neutral, and drying to obtain oxidized carbon nano tubes;

(2) Dispersing the carbon oxide nanotubes obtained in the step (1) in a mixed solution of water and a cosolvent, dropwise adding ammonia water to adjust the pH value of the solution to 8-10, and carrying out ultrasonic oscillation at room temperature to obtain a uniformly dispersed carbon oxide nanotube dispersion;

(3) Preparation of silica coated carbon nanotubes: slowly and dropwise adding tetraethyl orthosilicate into the carbon oxide nanotube dispersion liquid obtained in the step (2) under the stirring state, controlling the ratio of the tetraethyl orthosilicate to the carbon oxide nanotube to be 5mL/1 g-50 mL/1g, continuing to magnetically stir at room temperature-60 ^o ℃ for 0.5-12 hours after the dripping, and obtaining the carbon nanotube coated with silicon dioxide after suction filtration, washing and vacuum drying for 8-24 hours after the reaction;

(4) Calcining the carbon nanotube coated with the silicon dioxide obtained in the step (3) in an air atmosphere at 700-1000 ℃ for 1-6 hours to obtain a silicon dioxide hollow nanotube;

(5) Dispersing the silica hollow nanotubes obtained in the step (4) in water or ethanol to prepare a dispersion liquid with the concentration of 1g/10 mL-1 g/25 mL;

(6) Dissolving chitosan in 1-3 vol.% of acetic acid aqueous solution to prepare a chitosan solution with the concentration of 0.5-5 wt.%, adding all the silica hollow nanotube dispersion liquid prepared in the step (5) into the chitosan solution, and fully mixing to obtain a uniform dispersion liquid;

(7) Casting the uniform dispersion liquid obtained in the step (6) on a clean glass plate to form a film, drying, cooling to room temperature and demoulding;

(8) And (3) soaking the dry film obtained in the step (7) in sulfuric acid solution with the concentration of 0.1-4 mol/L for 0.5-5 h at room temperature to crosslink the composite film, repeatedly washing the crosslinked film with deionized water, and drying to obtain the organic-inorganic composite proton exchange membrane.

7. The method for preparing an organic-inorganic composite medium temperature proton exchange membrane according to claim 6, wherein the strong oxidizing solution in the step (1) is 98wt% or more sulfuric acid or 65wt% or more nitric acid, or a mixed solution of the two according to any volume ratio.

8. The method for preparing an organic-inorganic composite medium temperature proton exchange membrane according to claim 6, wherein the cosolvent in the step (2) is any one or a mixture of two of methanol, ethanol, n-propanol, isopropanol and n-butanol.

9. The method for preparing an organic-inorganic composite medium-temperature proton exchange membrane according to claim 8, wherein the ratio of the carbon nano tube to the cosolvent is 1g/50 mL-1 g/500mL, and the volume ratio of the cosolvent to the water is 0.5:1-5:1.

10. The method for preparing an organic-inorganic composite medium temperature proton exchange membrane according to claim 6, wherein the silica hollow nanotubes in the step (6) are added in an amount of 0.5wt.% to 30wt.% of chitosan.

11. Use of an organic-inorganic composite medium temperature proton exchange membrane according to any one of claims 1-5 or prepared by a method according to any one of claims 6-10 in the preparation of a proton exchange membrane fuel cell.